CONTENTS
THE
POPULAR SCIENCE
MONTHLY
EDITED BY
J. McKEEN CATTELL
VOL. LVII
MAY TO OCTOBER, 1900
NEW YORK AND LONDON
McCLURE, PHILLIPS AND COMPANY
1900
Copyright, 1900,
By McCLURE, PHILLIPS AND COMPANY.
PROFESSOR R. S. WOODWARD,
President of the American Association for the Advancement of Science.
THE
POPULAR SCIENCE
MONTHLY.
AUGUST, 1900.
RHYTHMS AND GEOLOGIC TIME.[A]
By G. K. GILBERT,
UNITED STATES GEOLOGICAL SURVEY.
[A] Read to the American Association for the Advancement of Science, at New York, June 26, 1900, as the address of the retiring President.
Custom dictates that in complying with the rule of the association I shall address you on some subject of a scientific character. But before doing so I may be permitted to pay my personal tribute to the honored and cherished leader of whose loss we are so keenly sensible on this occasion. His kindly personality, the charm which his earnestness and sincerity gave to his conversation, the range of his accomplishment, are inviting themes; but it is perhaps more fitting that I touch this evening on his character as a representative president of this body. The association holds a peculiar position among our scientific organizations of national or continental extent. Instead of narrowing its meetings by limitations of subject matter or membership, it cultivates the entire field of research and invites the interest and coöperation of all. It is thus not only the integrating body for professional investigators, but the bond of union between these and the great group of cultured men and women—the group from whose ranks the professional guild is recruited, through whom the scientific spirit is chiefly propagated, and through whose interest scientific research receives its financial support. Its aims and form of organization recognize, what pure science does not always itself recognize, that pure science is fundamentally the creature and servant of the material needs of mankind, and it thus stands for what might be called the human side of science. Edward Orton, throughout his career as teacher and investigator, was conspicuous for his attention to the human side of science. His most abstract work was consciously for the benefit of the community, and he ever sought opportunity to make its results directly available. In promoting the interests of the people of his adopted State he incidentally accomplished much for a larger community by helping it to an appreciation of the essential beneficence of the scientific study of nature and man. As an individual he was a diligent and successful laborer in the field which the association cultivates, and when the association selected him as its standard-bearer it made choice of one who was peculiarly its representative.
The subject to which I shall invite your attention this evening is by no means novel, but might better be called perennial or recurrent; for the problem of our earth’s age seems to bear repeated solution without loss of vigor or prestige. It has been a marked favorite, moreover, with presidents and vice-presidents, retiring or otherwise, when called upon to address assemblies whose fields of scientific interest are somewhat diverse—for the reason, I imagine, that while the specialist claims the problem as his peculiar theme of study, he feels that other denizens of the planet in question may not lack interest in the early lore of their estate.
The difficulty of the problem inheres in the fact that it not only transcends direct observation, but demands the extrapolation or extension of familiar physical laws and processes far beyond the ordinary range of qualifying conditions. From whatever side it is approached, the way must be paved by postulates, and the resulting views are so discrepant that impartial onlookers have come to be suspicious of these convenient and inviting stepping stones.
That vain expectation may not be aroused, I admit at the outset that I have not solved the problem and shall submit to you no estimates. My immediate interest is in the preliminary question of the available methods of approach, and it leads to the consideration of the ways, or the classes of ways, in which the measurement of time has been accomplished or attempted.
Of the artificial devices employed in practical horology there are two so venerable that their origins are lost in the obscurity of legendary myth. These are the clepsydra and the taper. In the clepsydra advantage is taken of the approximately uniform rate at which water escapes through a small orifice, and time is measured by gaging the loss of water from a discharging vessel or the gain in a receiving vessel. The hour-glass is one of its latest forms, in which sand takes the place of water. The taper depends for its value as a timepiece on the approximate uniformity of combustion when the area of fuel exposed to the air is definitely regulated. It survives chiefly in the prayer stick and safety fuse, but the graduated candle is perhaps still used to regulate monastic vigils.
The pendulum, a comparatively modern invention, excelling the clepsydra and taper in precision, has altogether supplanted them as the servant of civilization. Its accuracy results from the remarkable property that the period in which it completes an oscillation is almost exactly the same, whatever the arc through which it swings. It regulates the movements not only of our clocks, watches and chronometers, but of barographs, thermographs and a great variety of other machines for recording events and changes in their proper order and relation in respect to time.
I must mention also a special apparatus invented by astronomers and called a chronograph. It consists ordinarily of a revolving drum about which a paper is wrapped and against which rests a pen. As the drum turns the pen draws a line on the paper. Through an electric circuit the pen is brought under the influence of a pendulum in such a way that at the middle of each swing of the pendulum the pen is deflected, making a mark at right angles to the straight line. The series of marks thus drawn constitutes a time scale. The electric arrangements are so made that the pen will also be disturbed in consequence of some independent event, such as the firing of a gun or the transit of a star; and the mark caused by such disturbance, being automatically platted on the time scale, records the time of the event.
No attempt has been made to characterize these various timepieces with fullness, because they are already well known to most of those present, and, in fact, the chief motive for giving them separate mention is that they may serve as the basis of a classification. In the use of the clepsydra and taper, time is measured in terms of a continuous movement or process; in the use of the pendulum time is measured in terms of a movement which is periodically reversed. The classification embodies the fundamental distinction between continuous motion and rhythmic motion.
Passing now from the artificial to the natural measures of time, we find that they are all rhythmic. It is true that the spinning of the earth on its axis is in itself a continuous motion, but it would yield no time measure if the earth were alone in space, and so soon as the motion is considered in relation to some other celestial body it becomes rhythmic. As viewed from, or compared with, a fixed star, the period of its rhythm is the sidereal day; compared with the sun, it is the solar day, nearly four minutes longer; and compared with the moon, it is the lunar day, still longer by 49 minutes. As the sun supplies the energy for most of the physical and all the vital processes of the earth’s surface, the rhythm of the solar day is impressed in multitudinous ways on man and his environment, and he makes it his primary or standard unit of time. He has arbitrarily divided it into hours, minutes and seconds, and in terms of these units he says that the length of the sidereal day is a little more than 23 hours, 56 minutes and 4 seconds, and the average length of the lunar day is a little less than 24 hours and 49 minutes. The lunar day finds expression in the tides and is of moment to maritime folk, but the sidereal is known only to astronomers.
Next in the series of our natural time units is the month, or the rhythmic period of the moon regarded as a luminary. By our savage ancestors, who credited the moon with powers of great importance to themselves, much use was made of this unit, but as progress in knowledge has shown that the influence of the satellite had been vastly overrated, less and less attention has been paid to the returning crescent, and it is only in ecclesiastic calendars that the chronology of civilization now recognizes the natural month. Its shadow survives, without the substance, in the calendar month; and the week possibly represents an early attempt to subdivide it.
In passing to our third natural unit, the year, we again encounter solar influence, and find the rhythm of the earth’s orbit echoed and reechoed in innumerable physical and vital vibrations. As the attitude of the earth’s axis inclines one hemisphere toward the sun for part of the year and the other hemisphere for the remainder, the whole complex drama of climate is annually enacted, and the sequence of man’s activities is made to assume an annual rhythm. The year is second only to the day as a terrestrial unit of duration; and as the day is man’s standard for the minute division of time, so the year is his standard for larger divisions, and the decade, the century and the millenium are its multiples.
But the rhythms of day and night, of summer and winter, are not the only tides in the affairs of men. At birth we are small, weak and dependent, we grow larger and stronger, we become mature and independent, and then by reproducing our kind we complete the cycle, which begins again with our children. The cycle of human life is the generation, a time unit of somewhat indefinite length and varying in phase from family to family, but holding a place, nevertheless, in human chronology.
Still less definite is the rhythm of hereditary rulership, progressing from vigor through luxury to degeneracy, and closing its cycle in usurpation; yet it makes an epoch in the life of a nation or empire, and so the dynasty is one of the units of the historian.
The generation and the dynasty are of waning importance in human chronology, and they can claim no connection with the problem of geologic time; but here again I have turned aside for a moment in order to illustrate a principle of classification. The daily rhythm of waking and sleeping, of activity and rest, does not originate with man, but is imposed on him by the rhythm of light and darkness, and that in turn springs from the turning of the earth in relation to the shining sun. The yearly rhythm of sowing and harvesting, of the fan and the furnace, does not originate with man, but is imposed on him by the rhythm of the seasons, and that in turn springs from certain motions of the earth in relation to the glowing sun. But the rhythm of the generation and the rhythm of the dynasty have origin in the nature of man himself. The rhythms of human chronology may thus be grouped according to source in two classes, the imposed and the original; and the same distinction holds for other rhythms. The lunar day is an original rhythm of the earth as seen from the moon; the ground swell is an original rhythm of the ocean; but the tide is an imposed rhythm of the ocean, being derived from the lunar day. The swing of the pendulum is an original rhythm, but the regular excursion of the chronograph pen, being caused by the swing of the pendulum, is an imposed rhythm.
In giving brief consideration to each of the more important ways by which the problem of the earth’s age has been approached, I shall mention first those which follow the action of some continuous process, and afterward those which depend on the recognition of rhythms.
The earliest computations of geologic time, as well as the majority of all such computations, have followed the line of the most familiar and fundamental of geologic processes. All through the ages the rains, the rivers and the waves have been eating away the land, and the product of their gnawing has been received by the sea and spread out in layers of sediment. These layers have been hardened into rocky strata, and from time to time portions have been upraised and made part of the land. The record they contain makes the chief part of geologic history, and the groups into which they are divided correspond to the ages and periods of that history. In order to make use of these old sediments as measures of time it is necessary to know either their thickness or their volume, and also the rate at which they were laid down. As the actual process of sedimentation is concealed from view, advantage is taken of the fact that the whole quantity deposited in a year is exactly equalled by the whole quantity washed from the land in the same time, and measurements and estimates are made of the amounts brought to the sea by rivers and torn from the cliffs of the shore by waves. After an estimate has been obtained of the total annual sedimentation at the present time, it is necessary to assume either that the average rate in past ages has been the same or that it has differed in some definite way.
At this point the course of procedure divides. The computer may consider the aggregate amount of the sedimentary rocks, irrespective of their subdivisions, or he may consider the thicknesses of the various groups as exhibited in different localities. If he views the rocks collectively, as a total to be divided by the annual increment, his estimate of the total is founded primarily on direct measurements made at many places on the continents, but to the result of such measurements he must add a postulated amount for the rocks concealed by the ocean, and another postulated amount for the material which has been eroded from the land and deposited in the sea more than once.
If, on the other hand, he views each group of rocks by itself, and takes account of its thickness at some locality where it is well displayed, he must acquire in some way definite conceptions of the rates at which its component layers of sand, clay and limy mud were accumulated, or else he must postulate that its average rate of accretion bore some definite ratio to the present average rate of sedimentation for the whole ocean. This course is, on the whole, more difficult than the other, but it has yielded certain preliminary factors in which considerable confidence is felt. Whatever may have been the absolute rate of rock building in each locality, it is believed that a group of strata which exhibits great thickness in many places must represent more time than a group of similar strata which is everywhere thin, and that clays and marls, settling in quiet waters, are likely to represent, foot for foot, greater amounts of time than the coarser sediments gathered by strong currents; and studying the formations with regard to both thickness and texture, geologists have made out what are called time ratios—series of numbers expressing the relative lengths of the different ages, periods and epochs. Such estimates of ratios, when made by different persons, are found to vary much less than do the estimates of absolute time, and they will serve an excellent purpose whenever a satisfactory determination shall have been made of the duration of any one period.
Reade has varied the sedimentary method by restricting attention to the limestones, which have the peculiarity that their material is carried from the land in solution; and it is a point in favor of this procedure that the dissolved burdens of rivers are more easily measured than their burdens of clay and sand.
An independent system of time ratios has been founded on the principle of the evolution of life. Not all formations are equally supplied with fossils, but some of them contain voluminous records of contemporary life; and when account is taken of the amount of change from each full record to the next, the steps of the series are found to be of unequal magnitude. Though there is no method of precisely measuring the steps, even in a comparative way, it has yet been found possible to make approximate estimates, and these in the main lend support to the time ratios founded on sedimentation. They bring aid also at a point where the sedimentary data are weak, for the earliest formations are hard to classify and measure. It is true that these same formations are almost barren of fossils, but biologic inference does not therefore stop. The oldest known fauna, the Eocambrian, does not represent the beginnings of life, but a well-advanced stage, characterized by development along many divergent lines; and by comparing Eocambrian life with existing life the paleontologist is able to make an estimate of the relative progress in evolution before and after the Eocambrian epoch. The only absolute blank left by the time ratios pertains to an azoic age which may have intervened between the development of a habitable earth crust and the actual beginning of life.
Erosion and deposition have been used also, in a variety of ways, to compute the length of very recent geologic epochs. Thus, from the accumulation of sand in beaches Andrews estimated the age of Lake Michigan, and Upham the age of the glacial lake Agassiz; and from the erosion of the Niagara gorge the age of the river flowing through it has been estimated. But while these discussions have yielded conceptions of the nature of geologic time, and have served to illustrate the extreme complexity of the conditions which affect its measurement, they have accomplished little toward the determination of the length of a geologic period; for they have pertained only to a small fraction of what geologists call a period, and that fraction was of a somewhat abnormal character.
Wholly independent avenues of approach are opened by the study of processes pertaining to the earth as a planet, and with these the name of Kelvin is prominently associated.
As the rotation of the earth causes the tides, and as the tides expend energy, the tides must act as a brake, checking the speed of rotation. Therefore the earth has in the past spun faster than now, and its rate of spinning at any remote point of time may be computed. Assuming that the whole globe is solid and rigid, and that the geologic record could not begin until that condition had been attained, there could not have been great checking of rotation since consolidation. For if there had been, it would have resulted in the gathering of the oceans about the poles and the baring of the land near the equator, a condition very different from what actually obtains. This line of reasoning yields an obscure outer limit to the age of the earth.
On the assumption that the globe lacks something of perfect rigidity, G. H. Darwin has traced back the history of the earth and the moon to an epoch when the two bodies were united, their separation having been followed by the gradual enlargement of the moon’s orbit and the gradual retardation of the earth’s rotation; and this line of inquiry has also yielded an obscure outer limit to the antiquity of the earth as a habitable globe.
One of the most elaborate of all the computations starts with the assumption that at an initial epoch, when the outer part of the earth was consolidated from a liquid condition, the whole body of the planet had approximately the same temperature; and that as the surface afterward cooled by outward radiation there was a flow of heat to the surface by conduction from below. The rate of this flow has diminished from that epoch to the present time according to a definite law, and the present rate, being known from observation, affords a measure of the age of the crust. The strength of this computation lies in its definiteness and the simplicity of its data; its weakness in the fact that it postulates a knowledge of certain properties of rock—namely, its fusibility, conductivity and viscosity—when subjected to pressures and temperatures far greater than have ever been investigated experimentally.
A parallel line of discussion pertains to the sun. Great as is the quantity of heat which that incandescent globe yields to the earth, it is but a minute fraction of the whole amount with which it continually parts, for its radiation is equal in all directions, and the earth is but a speck in the solar sky. On the assumption that this immense loss of heat is accompanied by a corresponding loss of volume, the sun is shrinking at a definite rate, and a computation based on this rate has told how many millions of years ago the sun’s diameter should have been equal to the present diameter of the earth’s orbit. Manifestly the earth can not have been ready for habitation before the passage of that epoch, and so the computation yields a superior limit to the extent of geologic time.
Before passing to the next division of the subject—the computations based on rhythms—a few words may be given to the results which have been obtained from the study of continuous processes. Realizing that your patience may have been strained by the kaleidoscopic character of the rapid review which has seemed unavoidable, I shall spare you the recitation of numerical details and merely state in general terms that the geologists, or those who have reasoned from the rocks and fossils, have deduced values for the earth’s age very much larger than have been obtained by the physicists, or those who have reasoned from earth cooling, sun cooling and tidal friction. In order to express their results in millions of years the geologists must employ from three to five digits, while the physicists need but one or two. When these enormous discrepancies were first realized it was seen that serious errors must exist in some of the observational data or else in some of the theories employed; and geologists undertook with zeal the revision of their computations, making as earnest an effort for reconciliation as had been made a generation earlier to adjust the elements of the Hebrew cosmogony to the facts of geology. But after rediscussing the measurements and readjusting the assumptions so as to reduce the time estimates in every reasonable way—and perhaps in some that were not so reasonable—they were still unable to compress the chapters of geologic history between the narrow covers of physical limitation; and there the matter rests for the present.
The rocks which were formed as sediments show many traces of rhythm. Some are composed of layers, thin as paper, which alternate in color, so that when broken across they exhibit delicate banding. In the time of their making there was a periodic change in the character of the mud that settled from the water. Others are banded on a larger scale; and there are also bandings of texture where the color is uniform. Many formations are divided into separate strata, as though the process of accretion had been periodically interrupted. Series of hard strata are often separated by films or thin layers of softer material. Strata of two kinds are sometimes seen to alternate through many repetitions. Borings in the delta of the Mississippi show soils and remains of trees at many levels, alternating with river silts. The rock series in which coal occurs are monotonous repetitions of shale and sandstone. Belgian geologists have been so impressed by the recurrence of short sequences of strata that they have based an elaborate system of rock notation upon it.
Passing to still greater units, the large aggregates of strata sometimes called systems show in many cases a regular sequence, which Newberry called a “circle of deposition.” When complete, it comprises a sandstone or conglomerate, at base, then shale, limestone, shale and sandstone. This sequence is explained as the result of the gradual encroachment, or transgression, as it is called, of the sea over the land and its subsequent recession.
In certain bogs of Scandinavia deep accumulations of peat are traversed horizontally by layers including tree stumps in such way as to indicate that the ground has been alternately covered by forest and boggy moss. The broad glaciers of the Ice age grew alternately smaller and larger—or else were repeatedly dissipated and reformed—and their final waning was characterized by a series of halts or partial readvances, recorded in concentric belts of ice-brought drift. Of these belts, called moraines of recession, Taylor enumerates seventeen in a single system.
In explanation of these and other repetitive series incorporated in the structure of the earth’s crust, a variety of rhythmic causes have been adduced; and mention will be made of the more important, beginning with those which have the character of original rhythms.
A river flowing through its delta clogs its channel with sediment, and from time to time shifts its course to a new line, reaching the sea by a new mouth. Such changes interrupt and vary sedimentation in neighboring parts of the sea. Storms of rain make floods, and each flood may cause a separate stratum of sediment. Storms of wind give destructive force to the waves that beat the shore, and each storm may cause the deposit of an individual layer of sediment. Varying winds may drive currents this way and that, causing alternations in sedimentation.
To explain the forest beds buried in the Mississippi silts it has been suggested that the soft deposits of the delta from time to time settled and spread out under their own weight. Various alternations of strata, and especially those of the coal measures, have been ascribed to successive local subsidences of the earth’s crust, caused by the addition of loads of deposit. It has been suggested also that land undergoing erosion may rise up from time to time because relieved of load, and the character of sediment might be changed by such rising. Subterranean forces, of whatever origin, seemingly slumber while strains are accumulating, and then become suddenly manifest in dislocations and eruptions, and such catastrophes affect sedimentation.
A more general rhythm has been ascribed to the tidal retardation of rotation and the resulting change of the earth’s form. If the body of the earth has a rather high rigidity, we should expect that it would for a time resist the tendency to become more nearly spherical, while the water of the ocean would accommodate itself to the changing conditions of equilibrium by seeking the higher latitudes. Eventually, however, the solid earth would yield to the strain and its figure become adjusted to the slower rotation, and then the mobile water would return. Thus would be caused periodic transgressions by the sea, occurring alternately in high and low latitudes.
Another general rhythm has been recently suggested by Chamberlin in connection with the hypothesis that secular variations of climate are chiefly due to variations of the quantity of carbon dioxide in the atmosphere.[B] The system of interdependent factors he works out is too complex for presentation at this time, and I must content myself with saying that his explanation of the moraines of recession involves the interaction of a peculiar atmospheric condition with a condition of glaciation, each condition tending to aggravate the other, until the cumulative results brought about a reaction and the climatic pendulum swung in the opposite direction. With each successive oscillation the momentum was less, and an equilibrium was finally reached.
[B] An attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis. Journ. Geol., Vol. VII., 1899.
Few of these original rhythms have been used in computations of geologic time, and it is not believed that they have any positive value for that purpose. Nevertheless, account must be taken of them, because they compete with imposed rhythms for the explanation of many phenomena, and the imposed rhythms, wherever established, yield estimates of time.
The tidal period, or the half of the lunar day, is the shortest imposed rhythm appealed to in the explanation of the features of sedimentation. It is quite conceivable that the bottom of a quiet bay may receive at each tide a thin deposit of mud which could be distinguished in the resulting rock as a papery layer or lamina. If one could in some way identify a rock thus formed, he might learn how many half-days its making required by counting its laminæ, just as the years of a tree’s age are learned by counting its rings of growth.
The next imposed rhythm of geologic importance is the year. There are rivers, like the Nile, having but one notable flood in each year, and so depositing annual layers of sediment on their alluvial plains and on the sea beds near their mouths. Where oceanic currents are annually reversed by monsoons, sedimentation may be regularly varied, or interrupted, once a year. Streams from a glacier cease to run in winter, and this annual interruption may give a definite structure to resulting deposits. It is therefore probable that some of the laminæ or strata of rocks represent years, but the circumstances are rarely such that the investigator can bar out the possibility that part of the markings or separations were caused by original rhythms of unknown period.
The number of rhythms existing in the solar system is very large, but there are only two, in addition to the two just mentioned, which seem competent to write themselves in a legible way in the geologic record. These are the rhythms of precession and eccentricity.
Because the earth’s orbit is not quite circular and the sun’s position is a little out of the center, or eccentric, the two hemispheres into which the earth is divided by the equator do not receive their heat in the same way. The northern summer, or the period during which the northern hemisphere is inclined toward the sun, occurs when the earth is farthest from the sun, and the northern winter occurs when the earth is nearest to the sun, or in that part of the orbit called perihelion. These relations are exactly reversed for the southern hemisphere. The general effect of this is that the southern summer is hotter than the northern, and the southern winter is colder than the northern. In the southern part of the planet there is more contrast between summer and winter than in the northern. The sun sends to each half the same total quantity of heat in the course of a year, but the difference in distribution makes the climates different. The physics of the atmosphere is so intricate a subject that meteorologists are not fully agreed as to the theoretic consequences of these differences of solar heating, but it is generally believed that they are important, involving differences in the force of the winds, in the velocity and course of ocean currents, in vegetation, and in the extent of glaciers.
Now, the point of interest in the present connection is that the astronomic relations which occasion these peculiarities are not constant, but undergo a slow periodic change. The relation of the seasons to the orbit is gradually shifting, so that each season in turn coincides with the perihelion; and the climatic peculiarities of the two hemispheres, so far as they depend on planetary motions, are periodically reversed. The time in which the cycle of change is completed, or the period of the rhythm, is not always the same, but averages 21,000 years. It is commonly called the precessional period.[C]
[C] Strictly speaking, 21,000 years is the period of the precession of the equinoxes as referred to perihelion; but the perihelion is itself in motion. As referred to a fixed star the precession of the equinoxes has an average period of about 25,700 years.
Assuming that the climates of many parts of the earth are subject to a secular cycle, with contrasted phases every 10,500 years, we should expect to find records of the cycle in the sediments. A moist climate would tend to leach the calcareous matter from the rock, leaving an earthy soil behind, and in a succeeding drier climate the soil would be carried away; and thus the adjacent ocean would receive first calcareous and then earthy sediments. The increase of glaciers in one hemisphere would not only modify adjacent sediments directly, but, by adding matter on that side, would make a small difference in the position of the earth’s center of gravity. The ocean would move somewhat toward the weighted hemisphere, encroaching on some coasts and drawing down on others; and even a small change of that sort would modify the conditions of erosion and deposition to an appreciable extent in many localities.
Blytt ascribed to this astronomic cause the alternations of bog and forest in Scandinavia, as well as other sedimentary rhythms observed in Europe; and it has seemed to me competent to account for certain alternations of strata in the Cretaceous formations of Colorado. Croll used it to explain interglacial epochs, and Taylor has recently applied it to the moraines of recession.
The remaining astronomic rhythm of geologic import is the variation of eccentricity. At the present time our greatest distance from the sun exceeds our least distance by its thirtieth part, but the difference is not usually so small as this. It may increase to the seventh part of the whole distance, and it may fall to zero. Between these limits it fluctuates in a somewhat irregular way, in which the property of periodicity is not conspicuous. The effect of its fluctuation is inseparable from the precessional effect, and is related to it as a modifying condition. When the eccentricity is large the precessional rhythm is emphasized; when it is small the precessional effect is weak.
The variation of eccentricity is connected with the most celebrated of all attempts to determine a limited portion of geologic time. In the elaboration of the theory of the Ice age which bears his name, Croll correlated two important epochs of glaciation with epochs of high eccentricity computed to have occurred about 100,000 and 210,000 years ago. As the analysis of the glacial history progresses, these correlations will eventually be established or disproved, and should they be established it is possible that similar correlations may be made between events far more remote.
The studies of these several rhythms, while they have led to the computation of various epochs and stages of geologic time, have not yet furnished an estimate either of the entire age of the earth or of any large part of it. Nevertheless, I believe that they may profitably be followed with that end in view.
The system of rock layers, great and small, constituting the record of sedimentation, may be compared to the scroll of a chronograph. The geologic scroll bears many separate lines, one for each district where rocks are well displayed, but these are not independent, for they are labeled by fossils, and by means of these labels can be arranged in proper relation. In each time line are little jogs—changes in kind of rock or breaks in continuity—and these jogs record contemporary events. A new mountain was uplifted, perhaps, on the neighboring continent, or an old uplift received a new impulse. Through what Davis calls stream piracy a river gained or lost the drainage of a tract of country. Escaping lava threw a dam across the course of a stream, or some Krakatoa strewed ashes over the land and gave the rivers a new material to work on. The jogs may be faint or strong, many or few, and for long distances the lines may run smooth and straight; but so long as the jogs are irregular they give no clue to time. Here and there, however, the even line will betray a regularly recurring indentation or undulation, reflecting a rhythm and possibly significant of a remote pendulum whose rate of vibration is known. If it can be traced to such a pendulum there will result a determination of the rate at which the chronograph scroll moved when that part of the record was made; and a moderate number of such determinations, if well distributed, will convert the whole scroll into a definite time scale.
In other words, if a sufficient number of the rhythms embodied in strata can be identified with particular imposed rhythms, the rates of sedimentation under different circumstances and at different times will become known, and eventually so many parts of geologic time will have become subject to direct calculation that the intervals can be rationally bridged over by the aid of time ratios.
For this purpose there is only one of the imposed rhythms of practical value, namely, the precessional; but that one is, in my judgment, of high value. The tidal rhythm can not be expected to characterize any thick formation. The annual is liable to confusion with a variety of original rhythms, especially those connected with storms. The rhythm of eccentricity, being theoretically expressed only as an accentuation of the precessional, can not ordinarily be distinguished from it. But none of these qualifications apply to the precessional. It is not liable to confusion with the tidal and annual because its period is so much longer, being more than 2,000 times that of the annual. It has an eminently practical and convenient magnitude, in that its physical manifestation is well above the microscopic plane, and yet not so large as to prevent the frequent bringing of several examples into a single view. It is also practically regular in period, rarely deviating from the average length by more than the tenth part.
From the greater number of original rhythms it is distinguished, just as from the annual and tidal, by magnitude. The practical geologist would never confuse the deposit occasioned by a single storm, for example, with the sediments accumulated during an astronomic cycle of 20,000 years. But there are other original rhythms, known or surmised, which might have magnitudes of the same general order, and to discriminate the precessional from these it is necessary to employ other characters. Such characters are found in its regularity or evenness of period, and in its practical perpetuity. The diversion of the mouth of a great river such as the Hoang Ho or the Mississippi might recur only after long intervals; but from what we know of the behavior of smaller streams we may be sure that such events would be very irregular in time as well as in other ways. The intervals between volcanic eruptions at a particular vent or in a particular district may at times amount to thousands of years, but their irregularity is a characteristic feature. The same is true of the recurrent uplifts by which mountains grow, so far as we may judge them by the related phenomena of earthquakes; and the same category would seem to hold also the theoretically recurrent collapse of the globe under the strains arising from the slowing of rotation. The carbon-dioxide rhythm, known as yet only in the field of hypothesis, is hypothetically a running-down oscillation, like the lessening sway of the cradle when the push is no longer given.
But the precessional motion pulses steadily on through the ages, like the swing of a frictionless pendulum. Its throb may or may not be caught by the geologic process which obtains in a particular province and in a particular era, but whenever the conditions are favorable and the connection is made, the record should reflect the persistence and the regularity of the inciting rhythm.
The search of the rocks for records of the ticks of the precessional clock is an out-of-door work. Pursued as a closet study it could have no satisfactory outcome, because the printed descriptions of rock sequences are not sufficiently complete for the purpose; and the closet study of geology is peculiarly exposed to the perils of hobby-riding. A student of the time problem cannot be sure of a persistent, equable sedimentary rhythm without direct observation of the characters of the repeated layers. He needs to avail himself of every opportunity to study the series in its horizontal extent, and he should view the local problem of original versus imposed rhythm with the aid of all the light which the field evidence can cast on the conditions of sedimentation.
Neither do I think of rhythm seeking as a pursuit to absorb the whole time and energy of an individual and be followed steadily to a conclusion; but hope rather that it may receive the incidental and occasional attention of many of my colleagues of the hammer, as other errands lead them among cliffs of bedded rocks. If my suggestion should succeed in adding a working hypothesis or point of view to the equipment of field geologists, I should feel that the search had been begun in the most promising and advantageous manner. For not only would the subject of rhythms and their interpretations be advanced by reactions from multifarious individual experiences, but the stimulus of another hypothesis would lead to the discovery of unexpected meanings in stratigraphic detail.
It is one of the fortunate qualities of scientific research that its incidental and unanticipated results are not infrequently of equal or even greater value than those directly sought. Indeed, if it were not so, there would be no utilitarian harvest from the cultivation of the field of pure science.
In advocating the adoption of a new point of view from which to peer into the mysterious past, I would not be understood to advise the abandonment of old stand-points, but rather to emulate the surveyor, who makes measurement to inaccessible points by means of bearings from different sides. Every independent bearing on the earth’s beginning is a check on other bearings, and it is through the study of discrepancies that we are to discover the refractions by which our lines of sight are warped and twisted. The three principal lines we have now projected into the abyss of time miss one another altogether, so that there is no point of intersection. If any one of them is straight, both the others are hopelessly crooked. If we would succeed we should not only take new bearings from each discovered point of vantage, but strive in every way to discover the sources of error in the bearings we have already attempted.
THE PHOTOGRAPHY OF SOUND WAVES.
By Professor R. W. WOOD,
UNIVERSITY OF WISCONSIN.
Any one who has stood near a large naval gun during its discharge, will, I think, be prepared to admit that the sound of the explosion affects not only the ears, but the whole body as well, which experiences something not unlike a sudden blow. This blow, or concussion, as it is generally termed, is merely the impact of the wave of compressed air, spreading out in all directions around the gun. In the case of ordinary sounds, the compression of the air in the wave is so slight that only the delicate auditory nerves respond to the impact, hence we naturally conclude that sounds are perceived only by the ear. When dealing with sounds of very great intensity, this notion must be somewhat modified, for they certainly can be felt as well as heard. In some extreme cases, in fact, the sensation of feeling may be stronger than that of hearing, as in the case of which I shall speak presently. Is it also possible that we can perceive sound through the medium of any other sense organ, say the eye? ‘To see a noise’ certainly sounds like an absurdity; yet under certain conditions, sound waves in air can be made as distinctly visible as the ripples on a pond surrounding the splash of a stone. That they are not seen under ordinary conditions does not justify us in assuming them to be invisible. We all know that the currents of hot air rising from a stove, while not usually conspicuous, can be made visible by properly regulating the illumination, as by looking along the surface of the stove towards a window. The hot air is visible because in its optical properties it is different from the cold air surrounding it. The rays of light, passing through the unequally heated portions of the air, are bent in different directions, causing a distortion of objects seen through the heated currents. What we see, strictly speaking, is not the hot air itself, but a wavering and swimming of the objects seen through it. Yet I think we are justified in saying that the eye perceives the hot air.
Now sound waves in air, which are merely regions where the air is somewhat compressed, differ in their optical properties from the uncompressed portions, just as the hot air differs from the cold. As the pictures illustrating this article testify, they may be seen and photographed under proper conditions of illumination as readily as solid objects. We must remember, in the first place, that a sound wave travels with a velocity something greater than a thousand feet a second, rather less than the speed of a modern rifle ball, yet ten times faster than the fastest express train. The wave, even if it were stationary, could be seen only by adjusting the illumination with far greater care than was necessary in the case of the hot air, and we consequently can easily understand why we never see the waves under ordinary conditions.
While it is true that laboratory appliances are generally required to render them visible, I should like at the outset to cite an example to show that in the case of very loud sounds occurring in the open air the wave can be perceived by the eye, without the aid of any apparatus whatever. I will quote from an article by Prof. C. V. Boys, which appeared in ‘Nature,’ June 24, 1897. Mr. Boys first cites the following letter from Mr. E. J. Ryves: “On Tuesday, April 6th, I had occasion, while carrying out some experiments with explosives, to detonate one hundred pounds of a nitro-compound. The explosive was placed on the ground in the center of a slight depression, and in order to view the effect, I stationed myself, at a distance of about three hundred yards, on the side of a neighboring-hill. The detonation was complete, and a hole was made in the ground five feet deep and seven feet in diameter. A most interesting observation was made during the experiment. The sun was shining brightly, and at the moment of detonation the shadow of the sound wave was most distinctly seen leaving the area of disturbance. I heard the explosion as the shadow passed me, and I could follow it distinctly in its course down the valley for at least half a mile; it was so plainly visible that I believe it would photograph well with a suitable shutter.”
Professor Boys at once made preparations for photographing the phenomenon at the first opportunity. On May 19th the experiment was made. One hundred and twenty pounds of a nitro-compound were exploded, and an attempt made to photograph the sound shadow, both with the camera and the kinematograph, the latter instrument designed and operated by Mr. Paul. Writing of the experiment, Professor Boys says: “On the day on which I was present, about one hundred and twenty pounds of a nitro-compound were detonated, and ten pounds of black powder were added to make sufficient smoke to show on the plate. As the growth of the smoke cloud is far less rapid than the expansion of the sound shadow, no confusion could result from this. At the time of the explosion my whole attention was concentrated upon the camera, and for the moment I had forgotten to look for the ‘Ryves ring,’ as I think it might be called; but it was so conspicuous that it forced itself upon my attention. I felt, rather than heard, the explosion at the moment that it passed. We stationed ourselves as near as prudence would allow, at a distance of one hundred and twenty yards, so that only about one third of a second elapsed between the detonation and the passage of the shadow. The actual appearance of the ring was that of a strong, black, circular line, opening out with terrific speed from the point of explosion as a center. It was impossible to judge of the thickness of the shadow; it may have been three feet, or it may have been more at first, and have gradually become less in thickness, or possibly in depth of shade.”
Unfortunately, Professor Boys’s apparatus did not work satisfactorily, but a most interesting series of pictures was secured by the kinematograph. This instrument had been constructed especially for taking pictures at a very high rate of speed, viz., eighty exposures a second, or four times the usual number. The sound wave appears in the first dozen pictures as a hazy ring of light, opening out from the center of explosion. The ring, though not very conspicuous when the pictures are viewed singly, becomes a striking object when they are projected in rapid succession on the screen. We see the rush of smoke along the ground to the box in which the explosion is confined (the smoke of the quick fuse); then comes the burst of the explosion with such startling reality that we involuntarily jump. The image of the sound wave flies out in the form of a white ring, and is gone in a moment; and there remain only the rolling clouds of smoke. It is interesting to observe the development of the explosion by running the machine quite slowly, and by thus magnifying time to follow the changes which ordinarily occur in such rapid succession that the eye is unable to perceive them.
Of this series of pictures, Professor Boys says: “The kinematograph fails to show any black ring; and this is not surprising, as with the exposure of about one one hundredth of a second the shadow would have to be at least eleven feet thick in order that some part should remain obscured during the whole exposure. As a fact, there is clearly seen a circular light shading, which does—so far as one can judge from the supposed rate of working and the known distances—expand at about the same rate as the observed shadow, but it is lighter than the ground and shaded, instead of being dark and sharp, as seen by the eye.”
Kinetoscope Film of Explosion.
So much for the visibility of sound under ordinary conditions. In the laboratory, by means of an optical contrivance due to the German physicist Toepler, we can secure a means of illumination so sensitive that the warm air rising from a person’s hand appears like dense black smoke. Moreover, since we are working on a small scale, we can use the electric spark as the source of light, and dispense with the photographic shutter. This is a great advantage, for the time of the exposure is, under these conditions, only about one fifty-thousandth of a second, during which time the sound wave will move scarcely a quarter of an inch. During the past year I have made a very complete series of photographs of sound waves, which illustrate in a most beautiful manner the fundamental principles of wave motion. It is not practicable to give here a full description of the apparatus used, but a brief outline may make the method intelligible. The sound photographed in each case is the crack of an electric spark, which is illuminated and photographed by the light of a second spark, occurring a brief instant later. In front of a large lens (a telescope objective, for example) two brass balls are mounted, between which the ‘sound spark,’ as I shall call it, passes. The instant the spark jumps across the gap, a spherical wave of condensed air starts out, which, when it reaches our ear, gives the sensation of a snap. The object is to photograph this wave before it gets beyond the limits of the lens. The camera is mounted in front of the lens and focussed on the brass balls, which appear in line in the picture, so that the sound spark is always hidden by the front one. The spark, on jumping between the balls, charges a Leyden jar, which instantly discharges itself between two wires placed behind the lens, producing the illuminating spark. This second spark can be made to lag behind the first just long enough to catch the sound wave when it is but a few inches in diameter, notwithstanding the fact that the spherical wave is expanding at the rate of eleven hundred feet a second. The photographs show in every case the circle of the lens filled up with the light of the illuminating spark, the brass balls (in line) and the rods that support them, and the sound wave, which appears in the simplest case as a circle of light and shade surrounding the balls. By placing an obstacle in the way of the wave we get the reflected wave or echo, and we shall see that the form of this echo may be very complicated.
Fig. 1. Sound Wave Reflected from a Plane Surface.
It will be well at the outset to remind the reader of the close analogy between sound and light. A burning candle gives out spherical light waves, just as the snapping sparks give out sound waves. The form of the reflected light wave will be identical with that of a sound wave reflected under similar conditions. As we can not see the light waves themselves, we can only determine their form by calculation, and it is interesting to see that the forms photographed are identical in every case with the calculated ones. The object in view was to secure acoustical illustrations of as many of the phenomena connected with light as possible. We will begin with the very simplest case of all: the reflection of a spherical sound wave from a flat surface, corresponding to the reflection of light from a plane mirror. It can be shown by geometry that the reflected wave or echo will be a portion of a sphere, the center of which lies as far below the reflecting surface as the point at which the sound originates is above it. In the case of light, this point constitutes the image in the mirror. Referring to the photograph, we see the reflected wave in three successive positions, the interval between the sound spark and the illuminating spark having been progressively increased. The brass balls are shown at A, and beneath them the flat plate B, which acts as a reflector. In the first picture the sound wave C appears as a circle of light and shade, and has just intersected the plate. The echo appears at D. In the next two pictures the original wave has passed out of the field, and there remains only the echo.
It may, perhaps, be not out of place to remind the reader of the relation between rays of light and the wave surface. What we term light rays have no real existence, the ray being merely the path traversed by a small portion of consecutive wave surfaces. Since the wave surface always moves in a direction perpendicular to itself, the rays are always normal to it. For instance, in the above case of a spherical wave diverging from a point, the rays radiate in all directions from the point; the same is true in the case of the echo, the rays radiating from the image point below the reflecting surface. In all subsequent cases the reader can, if interested in tracing the analogy between sound and light, draw lines perpendicular to the reflected wave surfaces representing the system of reflected waves.
We will now consider a second case of reflection. We know that if a lamp is placed in the focus of a concave mirror, the rays, instead of diverging in all directions, issue from the mirror in a narrow beam. The headlight of a locomotive and the naval searchlight are examples of the practical use made of this property. If the curvature of the mirror is parabolical, the rays leaving it are parallel; consequently mirrors of this form are employed rather than spherical ones. But what has the mirror done to the wave surface which is obviously spherical when it leaves the lamp, and what is its form after reflection? The wave surface, I have said, is always perpendicular to the rays; consequently in cases where we have parallel rays we should expect the wave to be flat or plane.
Fig. 2. Spherical Sound Wave.
Examine the second photograph, which shows a spherical, sound wave starting at the focus of a parabolic mirror. The echo appears as a straight line, instead of a circle as in the previous case, which shows us that the wave surface is flat.
If now our mirror is a portion of a sphere instead of a paraboloid, our reflected wave is not flat, and the reflected rays are not all parallel, the departure from parallelism increasing as we consider rays reflected from points farther and farther away from the center of the mirror. A photograph illustrating the reflection of sound under these conditions is next shown, the echo wave being shaped like a flat-bottomed saucer. As the saucer moves upward the curved sides converge to a focus at the edge of the flat bottom, disappearing for the moment (as is shown in the fourth picture of the series), and then reappearing on the under side after passing through the focus, the saucer turning inside out.
If, instead of having a hemisphere, as in the last case, we have a complete spherical mirror, shutting the wave up inside a hollow ball, we get exceedingly curious forms; for the wave can not get out, and is bounced back and forth, becoming more and more complicated at each reflection. This is illustrated in our next photograph, the mirror being a broad strip of metal bent into a circle.[D] Intricate as these wave surfaces are, they have all been verified by geometrical constructions, as I shall presently show.
[D] Cylindrical mirrors have been used instead of spherical, for obvious reasons. A sectional view of the reflected wave is the same in this case as when produced by a spherical surface.
Another very interesting case of reflection is that occurring inside an elliptical mirror. When light diverges from one of the two foci of such a mirror, all the rays are brought accurately to the other focus. If rays of light come to a focus from all directions, it is evident that the wave surface must be a sphere, which, instead of expanding, is collapsing. This is very beautifully shown in the photographs. The sound wave starts in one focus and the reflected wave, of spherical form also, shrinks to a point at the other focus. (See [Fig. 5].)
Fig. 3. A Wave Reflected from a Portion of a Sphere.
Fig. 4. A Wave from a Cylindrical Mirror.
In the next series the wave starts outside of the field of the lens, and enters a hemispherical mirror. We know that a concave mirror has the power of bringing light to a focus at a point situated half-way between the surface of the mirror and its center of curvature. If the light comes from a very distant point, and the mirror is parabolic in form, the rays are brought accurately to a focus; which means that the reflected wave is a converging sphere,—a condition the opposite of that in which spherical waves start in the focus of such a mirror. If, however, the mirror is spherical, only a portion of the light comes to a focus. On examining the pictures we see that the reflected wave has a form resembling a volcanic cone with a bowl-shaped crater. See the third and fourth pictures of the series. The bowl of the crater shrinks to a point half-way between the surface of the mirror and its center of curvature, and represents that portion of the light which comes to a focus, while the sides of the cone run in under the collapsing bowl, and eventually cross. (No. 6 of the series.) From now on the portion which has come to a focus diverges, uniting with the sides of the cone, the whole passing out of the mirror in the form of a horseshoe.
Fig. 5. A Wave from an Elliptical Mirror.
Fig. 6. A Wave Starting Outside the Field of the Lens.
Fig. 7. A Case of Refraction.
We will now consider a case of refraction, and show the slower velocity of the sound wave in carbonic acid. A narrow glass tank, covered with an exceedingly thin film of collodion, was filled with the heavy gas and placed under the brass balls. When the sound wave strikes the collodion surface, it breaks up into two components, one reflected back into the air, the other transmitted down through the carbonic acid. An examination of the series shows that the reflected wave in air has moved farther from the collodion film than the transmitted wave, which, as a matter of fact, has been flattened out into a hyperboloid. Exactly the same thing happens when light strikes a block of glass. We have rays reflected from the surface, and rays transmitted through the block, the waves which give rise to the latter moving slower than the ones in air.
A complete discussion of all of the cases that have been studied in this way would probably prove wearisome to the general reader. Prisms and lenses of collodion filled with carbonic acid and hydrogen gas have been made, and their action on the wave surface photographed. Diffraction, or the bending of the waves around obstacles, and the very complicated effects when the waves are reflected from corrugated surfaces, are also well shown. I shall, however, omit further mention of them and speak of but one other case, possibly the most beautiful of all.
Fig. 8. A Musical Tone.
In all the cases that we have considered, it must be remembered that we have been dealing with a single wave—a pulse, as it is called. Musical tones are caused by trains of waves, the pitch of the note corresponding to the distance between the waves, or to the rate at which the separate pulses beat upon the drum of the ear. For studying the changes produced by reflection, wave trains would have been useless, owing to the confusion which would have resulted from the superposition of the different waves. Moreover, it is doubtful whether an ordinary musical tone could be photographed in this way; for the distance between the waves, even in the shrillest tones, is four or five inches, and the abrupt change in density, necessary for the perception of the wave, is not present. It is possible, however, to create a wave train or musical tone which can be photographed. The reader may perhaps have noticed that on a very still night, when walking beside a picket fence or in front of a high flight of steps, the sounds of his footsteps are echoed from the palings as metallic squeaks. Each picket, as the single wave caused by the footfall sweeps along the fence, reflects a little wave; consequently a train of waves falls on the ear, the distance between the waves corresponding to the distance between the pickets. The closer together the pickets, the shriller the squeak. In point of fact, the distance between the waves in such a train is twice the distance between the palings, since they are not struck simultaneously by the footstep wave, but in succession.
This phenomenon, of the creation of a musical tone by the reflection of a noise, was reproduced by reflecting the crack of the spark from a little flight of steps. In the first picture the wave is seen half way between its origin and the reflecting surface. In the second it has struck the top stair, which is giving off its echo, the first wave of our artificially constructed musical tone. In the third we find the original wave at the sixth step, with a well-developed train of five waves rising from the flight. The following three pictures show the further development of the wave train. The height of each step was about a quarter of an inch; consequently the distance between the waves was half an inch. This would correspond to a note about three octaves above the highest ever used in music.
Fig 9. The Reflection Inside the Hollow Sphere.
While experimenting with the complete circular mirror, which, it will be remembered, gave the most complicated forms, it occurred to me that a very vivid idea of how these curious wave surfaces are produced could be obtained by preparing a complete series in proper order on a kinetoscope film, and then projecting them in succession on the screen. The experimental difficulties were, however, too great to make it seem worth while to attempt to obtain a series of pictures of the actual waves, it being very difficult to accurately regulate the time interval between the two sparks. The easier method of making a large number of geometrical constructions, and then photographing them in succession on the film, was accordingly adopted. Three complete sets of drawings, to the number of about one hundred each, were prepared for three separate cases of reflection;—viz.: the entrance of a plane wave into a hemispherical mirror, the passage of a spherical wave out from the focus of a hemispherical mirror, and the multiple reflection of a spherical wave inside of a complete spherical mirror. Special methods were devised for simplifying the constructions, and much less labor was required in the preparation of the diagrams than one would suppose. The results fully justified the labor, the evolutions of the waves being shown in a most striking manner. These films I exhibited before the Royal Society in February last, and a more complete description of the manner of preparing them may be found in the Proceedings of the Society.
A portion of one of these series is reproduced, about one in four or five of the separate diagrams being given. The series runs from left to right in horizontal rows. When projected on the screen, the spherical wave is seen gradually to expand from the focus point, like a swelling soap bubble; it strikes the surface, and the bowl-shaped echo bounces off and follows the unreflected portion across the field; these two portions are then reflected in turn, and the curiously looped wave flies back and forth across the mirror, changing continuously all the time, and becoming more complicated at each reflection. These diagrams should be compared with the photographs shown in the fourth series.
One must not suppose that these beautiful forms exist only in the laboratory. Every time we speak, spherical waves bounce off the floor, ceiling and walls of the room, while in any ordinary bowl or basin the curious crater-shaped echoes are formed. Glance once more at the wave surfaces produced within a hollow sphere, and try to imagine the complexity of the aerial vibrations caused by a fly buzzing around in an empty water-caraffe! The photographs enable us to realize what is going on around us all the time—this our perceptions are fortunately too dull to perceive. Life would be a nightmare if we were obliged to see the myriads of flying sound waves bounding and rebounding about us in every direction, and combining into grotesque and ever-changing forms. It is just as well, on the whole, that the light of the electric spark and the delicate optical device of Toepler are necessary to bring them into view.
THE PSYCHOLOGY OF RED.
By HAVELOCK ELLIS.
Among all colors, the most poignantly emotional tone undoubtedly belongs to red. The ancient observation concerning the resemblance of scarlet to the notes of a trumpet has often been repeated, though it was probably unknown to the young Japanese lady who, on hearing a boy sing in a fine contralto voice, exclaimed: “That boy’s voice is red.” On the one hand, red is the color that idiots most easily learn to recognize; on the other hand, Kirchhoff, the chemist, called it the most aristocratic of colors; Pouchet, the zoölogist, was inclined to think that it was a color apart, not to be paralleled with any other chromatic sensation, and recalled that the retinal pigment is red; Laycock, the physician, confessed that he preferred the gorgeous red tints of an autumn sunset to either musical sounds or gustatory flavors. Artists more cautious than men of science in expressing such a preference—knowing that a color possesses its special virtue in relation to other colors, and that all are of infinite variety—yet easily reveal, one may often note, a predilection for red by introducing it into scenes where it is not naturally obvious, whether we turn to a great landscape painter like Constable or to a great figure painter like Rubens, who, with the development of his genius, displayed even greater daring in the introduction of red pigments into his work.
In all parts of the world red is symbolical of joyous emotion. Often, either alone or in association with yellow, occasionally with green, it is the fortunate or sacred color. In lands so far apart as France and Madagascar scarlet garments were at one time the exclusive privilege of the royal family. A great many different colors are symbolical of mourning in various parts of the world; white, gray, yellow, brown, blue, violet, black can be so used, but, so far as I am aware, red never. Everywhere we find, again, that red pigments and dyes, and especially red ochre, are apparently the first to be used at the beginning of civilization, and that they usually continue to be preferred even after other colors are introduced. There is indeed one quarter of the globe where the allied color of yellow, which often elsewhere is the favorite after red, may be said to come first. In a region of which the Malay peninsula is the center and which includes a large part of China, Burmah and the lower coast of India, yellow is the sacred and preferred color, but this is the only large district which presents us with any exception to the general rule, among either higher or lower races, and since yellow falls into the same group as red, and belongs to a neighboring part of the spectrum, even this phenomenon can scarcely be said to clash seriously with the general uniformity.[E]
[E] A further partial exception is furnished by the tendency to prefer green which may be found in certain countries, now or formerly Mahommedan, such as North Africa and to a large extent Spain, which have an arid and more or less desert climate.
If we turn to Australia, whither the anthropologist often turns in order to explore some of the most primitive and undisturbed data of early human culture still available for study, we find the preference for red very well marked. In times of rejoicing the tribes at Port Mackay, Curr remarked, paint themselves red; in times of mourning, white. In describing the paintings and rock carvings of the Australians, Mathews states that red, white, black and occasionally yellow pigments were used, precisely the four pigments which Karl von den Steinen found in use in Central Brazil. Prof. Baldwin Spencer and Mr. Gillen, in their valuable work on the natives of Central Australia, have pointed out the significance and importance of red ochre. One of the most striking and characteristic features, they say, of Central Australians’ implements and weapons is the coating of red ochre with which the native covers everything except his spear and spear-thrower. The hair is greased and red-ochred, and red ochre is the most striking feature in decoration generally. For ages past the Australian native has been accustomed to rub this substance regularly over his most sacred objects, and then over ordinary objects.
There is, however, no need to go so far afield in order to illustrate the primitive use of red ochre. Our own European ancestors followed exactly the same methods, and the German woman of early ages used red and yellow ochre to adorn her face and body, while the finds of the ice age at Schussenquelle, described by Fraas, included a brilliant red paste (oxide of iron with reindeer fat) evidently intended for purposes of adornment. Moreover, the early artists of classic times had precisely the same predilections in color as the aboriginal Australian artists. Red, white, black and yellow are the dominant colors in the Iliad, and Pliny mentions that the most ancient pictures were painted in various reds, while at a later date red and yellow predominated. He also mentions that yellow was the favorite color of women for garments, and was specially used at marriages, while red being a sacred color and apt to provoke joy, was used at popular festivals, in the form of minium and cinnabar, to smear the statues of Jupiter.
This well-nigh universal recognition of the peculiarly intense emotional tone of red is reflected in language. The color words of civilized and uncivilized peoples have been investigated with interesting and on the whole remarkably harmonious results. It is only necessary here to refer to them briefly in so far as they are related to our present subject. It seems that in every country the words for the colors at the red end of the spectrum are of earlier appearance, more definite and more numerous, than for those at the violet end. On the Niger it appears that there are only three color words, red, white and black, and everything that is not white or black is called red. The careful investigation of the natives of Torres Straits and New Guinea by Dr. W. H. R. Rivers, of the Cambridge Anthropological Expedition, has shown that at Murray Island, Mabuiag and Kiwai there were definite names for red, less definite for yellow, still less so for green, while any definite name for blue could not be found. In this way as we pass from the colors of long wave-length towards those of short wave-length we find the color nomenclature becoming regularly less definite. In Kiwai and Murray Island the same word was applied to blue and black, and at Mabuiag there was a word (for sea-color) which could be applied either to blue or green, while Australian natives from Fitzroy River seemed limited to words for red, white and black. In a neighboring region of Northern Queensland Dr. Walter Roth has reached almost identical results, the tribes having distinct names for red and yellow, as applied to ochre, while blue is confounded in nomenclature with black. In Brazil, again, while all tribes use separate words for red, yellow, white and black, only one had a word for blue and green. Even so æsthetic a people as the Japanese have no general words for either blue or green, and apply the same color word to a green tree and the unclouded sky.
Here again we may trace similar phenomena in Europe; the same greater primitiveness, precision and copiousness of the color vocabulary at the long wave end of the spectrum are found among Europeans as well as among the lowest savages. The vagueness of the Greek color vocabulary, especially at the violet end of the spectrum, has led to much controversy. Latin was especially rich in synonyms for red and yellow, very poor in synonyms for green and blue. The Latin tongue had even to borrow a word for blue from Teutonic speech; caeruleus originally meant dark. Even in the second century A. D. Aulus Gellius, who knew seven synonyms for red and yellow, scarcely mentions green and blue. Magnus has pointed out that a preference for the colors at the violet end of the spectrum coincided with the spread of Christianity, to which we owe it, he believes, that yellow ceased to be popular and was treated with opprobrium.[F] Modern English bears witness that our ancestors, like the Homeric poets, resembled the Australian aborigines in identifying the color of the short wave end of the spectrum with entire absence of color, for ‘blue’ and ‘black’ appear to be etymologically the same word.
[F] In this connection I may mention that the preference for green, which, as I have shown elsewhere (“The Color Sense in Literature,” Contemporary Review, May, 1896), developed in English literature with the rise of Puritanism in the seventeenth century.
At this point we come across an interesting and once warmly debated question. It was maintained some twenty years ago by writers who had been impressed by the defectiveness of the color vocabulary at the short wave-length end of the spectrum, that primitive man generally, and early Hellenic man in particular, were insensitive to the colors at that end of the spectrum, and unable to distinguish them. On investigation of individuals belonging to savage races it appeared, however, that no marked inferiority in color discrimination could be demonstrated. Hence it became clear that the vague and defective vocabulary for blue and green must be due to some other cause than vague and defective perception, and that sensation and nomenclature were not sufficiently parallel to enable us to argue from one to the other.
That, in the main, is a conclusion which still holds good. In all parts of the world it has been found that color discrimination, even amongst the lowest savages, is far more accurate than color nomenclature. Thus of an African Bantu tribe, the Mang’anja, Miss Werner states that they can discriminate all varieties of blue in beads, but call them all black. The sky is black; so is any green, brown or grey article, though a very bright grey counts as white. Violet or purple is black. Yellow is either red or white. A word supposed sometimes to mean green really means raw, unripe or even wet. Thus the Mang’anja only have three colors—black, white and red. In quite a different region, the Zulus, more advanced in color nomenclature, have not only black, white and red, but a word which may mean either green or blue, and another which means yellow, buff or grey, or some shade of brown. At the same time it now appears that the earlier scientific writers on this subject were not entirely wrong in stating that among savages there is some actual failure of perception at the short wave end of the spectrum, although they were wrong in arguing that it was necessarily involved in the defects of color vocabulary, and in imagining that it could be as extensive as that hypothesis demanded. It now appears that the conclusions reached by Hugo Magnus of Breslau, as expressed in 1883 in his study ‘Ueber Ethnologische Untersuchungen des Farbensinnes,’ fairly answer to the facts. In large measure relying on the examination of 300 Chukchis made by Almquist during the Nordenskiold Expedition, Magnus concluded that although the color vision of the uncivilized has the same range from red to violet as that of the civilized and all the colors can usually be separately distinguished, there is sometimes a certain dullness, a diminished energy of sensation, as regards green and blue, the shorter and more refrangible waves of the spectrum, while the colors at the other end are perceived with much greater vividness. Stephenson, more recently, among over one thousand Chinese, examined at various places, found only one case of color blindness, but a frequent tendency to confuse green and blue and also blue and purple, while Dr. Adele Fielde, of Swatow, China, among 1,200 Chinese of both sexes examined by Thomson’s wool test, found that more than half mixed up green and blue, and many even seemed to be quite blind to violet. Ernest Krause also has argued that primitive man was most sensitive to the red end of the spectrum, hence setting about to obtain red pigments and acquiring definite names for them, an explanation which is accepted by Karl von den Steinen to account for the phenomena among the Central Brazilians. The recent investigations of Rivers at Torres Straits have confirmed the conclusions of Magnus. He found that, corresponding to the defect of color terminology, though to a much less degree, there appeared to be an actual defect of vision for colors of short wave-length; in testing with colored wools no mistake was ever made with reds, but blues and greens were constantly confused, as were blue and violet.
It may even be argued that the same defect exists to a minor degree not only among the peoples of Eastern Asia whose æsthetic sense is highly developed, but among civilized Europeans when any kind of color blindness is altogether excluded. This was noted long since by Holmgren, who remarked that some persons, though able to distinguish between blue and green wools when placed together, were liable to call the blue wool green, and the green blue, when they saw them separately. Magnus also showed that such an inability is apt to appear at a very early stage in some persons when the illumination is diminished, although the perception of red and yellow remains perfectly distinct. He further showed that blue and green at certain distances are often much more difficult to recognize than red. Most people probably are conscious of difficulty in distinguishing blue and green pigments with diminished light and find that blue easily passes into black. Violet also appears for many people to be merely a variety of blue; the word itself, we may note, is recent in our language, and plays a very small part in our poetic literature, and in fact the color itself, if we rigidly exclude purple, is extremely rare in nature. It is a noteworthy fact in this connection that in normal persons the color sense may be easily educated; this is not merely a fact of daily observation, but has been exactly demonstrated by Féré, who by means of his chromoptoscopic boxes, containing very dilute colored solutions, found that with practice it was possible to recognize solutions which had previously seemed uncolored. It is also noteworthy that in the achromatopsia of the hysterical, as Charcot showed and as Parimand has since confirmed, the order in which the colors usually disappear is violet, green, blue, red; sometimes the paradoxical fact is found that red will give a luminous sensation in a contracted visual field when even white gives no luminous sensation. This persistence of red vision in the hysterical is only one instance of a predilection for red which has often been noted as very marked among the hysterical. Red also exerted a great fascination over the victims of the mediæval hysterical epidemics of tarantism in Italy, while the victims of the German mediæval epidemic of St. Vitus’s dance imagined that they were immersed in a stream of blood which compelled them to leap up.
It may be noted that red and perhaps yellow have been stated to be the only colors visible in dreams; this is possibly due to the blood-vessels. Such an explanation is probable with regard to the various subjective visual sensations which constitute an aura in epilepsy, among which, as Gowers notes, red and reddish yellow are most frequently found. Féré has further noted that in various emotional states somewhat resembling epilepsy, and even in mystic exaltation, red may be subjectively seen. Simroth has gone so far as to argue that not only is red fundamental in human color psychology, but that in living organisms generally, even as a pigment, red is the most primitive of colors, that since the algæ at the greatest sea-depths are red it is possible that protoplasm at first only responded to rays of long wave-length, and that with increased metabolism colors became differentiated, following the order in the spectrum.
If it is really the case that in the evolution of the race familiarity with the red end of the spectrum has been earlier and more perfectly acquired than with the violet end, and that red and yellow made a more profound impression on primitive man than green and blue, we should expect to find this evolution reflected in the development of the individual, and that the child would earlier acquire a sensitiveness for red and orange and yellow than for green and blue and violet. This seems actually to be the case. The study of the color sense in children is, indeed, even more difficult than in savages; and many investigators have probably succumbed to the fallacies involved in this study. Doubtless we may thus account for some discrepancies in the attempts to ascertain the facts of color perception and color preference in children, while doubtless also there are individual differences which discount the value of experiments made on only a single child. A few careful and elaborate investigations, however, especially that of Garbini on 600 North Italian children of various ages, have thrown much light on the matter. There is fairly general agreement that red is the first color that attracts young children and which they recognize. That is the result recorded by Uffelmann in Germany, while Preyer found yellow and red at the head; Binet in France concluded that red comes first; Wolfe in America reached the same result, and Luckey noted that his own children seemed to enjoy red, orange and yellow very much earlier than they could perceive blue, which seemed to come last. Baldwin, indeed, found in the case of his own child that blue seemed more attractive than red; his methods have, however, been criticised, and his experiments failed to include yellow. Mrs. Moore found that her baby, between the sixteenth and forty-fifth weeks, nearly always preferred a yellow ball to a red ball; this was doubtless not a matter of color, but of brightness, for there is no reason to suppose chromatic perception at so early an age. Red, orange and yellow, it may be added, are perceived by a slightly lower illumination than green, blue and violet, the last being the most difficult of all to perceive, so that it is not surprising that the colors at the violet end should be inconspicuous to young infants. Garbini, whose experiments are worth noting in more detail, found that the order of perception is red, green, yellow, orange, blue and violet, and as he experimented with a large number of children and used methods which so competent a judge as Binet regards as approaching perfection, his results may be considered a fair approach to the truth. He found that for the first few days after birth the infant shuns the light; then, about the fourteenth day, he ceases to be photophobic and begins to enjoy the light, as is shown by his being quieted when brought into a bright light and crying when taken from it; this may sometimes begin even about the fifth day. Between the fifth week and the eighteenth month children show signs of distinguishing white, black and grey objects. It is not until after the eighteenth month that their chromatic perception begins, any preference for red and yellow objects at an earlier age being due merely to their greater luminosity. Garbini considers that it is the center of the retina, or the portion most sensitive to red and yellow, which is most exercised in young infants. Between the second and third years children, both boys and girls, were found to be most successful in the recognition of red, then of green, but they very often confused orange with red, and mixed up yellow, blue, violet and green; he thinks they tend to confuse a color with the preceding color in spectral order. Under the age of three children may be said to be color-blind, and they are liable to confuse rosy tints with green. Between the ages of three and five they are able to distinguish red in any gradation, green nearly always, with an occasional confusion with red, while yellow is sometimes confused with orange, orange sometimes replaced by rose, blue often not recognized in its gradations, and violet often selected in place of blue. At this age, also (as in hysterical anæsthesia of the retina), blue seems dark or black. In the fifth and sixth years red, green and yellow are always correctly chosen; orange gradations are not always recognized, and blue and violet come last, being sometimes confused. In the sixth year children are perfecting their knowledge of orange, blue and violet and completing their knowledge of color designations. Garbini has reached the important result that color perceptions and verbal expression of the perceptions follow exactly parallel paths, so that in studying verbal expression we are really studying perception, with the important distinction that the expression comes much later than the perception.[G] These investigations of Garbini are very significant, and there can be little doubt that the evolution of the child’s color sense repeats that of the race.
[G] Garbini, “Evoluzione del senso cromatico nella infanzia,” Archivio per l’Antropologia, 1894. I.
In dealing with the color perceptions of savages and children we are, of course, to some extent dealing more or less unconsciously with their color preferences. There is some interest from our present point of view in considering the conscious color preferences of young and adult civilized persons. Red, as we have seen, is the color that fascinates our attention earliest, that we see and recognize most vividly; it remains the color that attracts our attention most readily and that gives us the greatest emotional shock. It by no means necessarily follows that it is the most pleasurable color. As a matter of fact, such evidence as is available shows that very often it is not. There seems reason to think that after the first early perception of red, and early pleasure in it, yellow or orange is frequently the favorite color, the preference often lasting during several years of childhood; Preyer’s child liked and discriminated yellow best, and Miss Shinn was inclined to think that it was the favorite color of her niece, who in the twenty-eighth month showed a special fondness for daffodils and for a yellow dress. Barnes found that in children the love of yellow diminishes with age. Binet’s child was specially preoccupied with orange. Aars in an elaborate and frequently varied investigation into the color preferences of eight children (four of each sex), between four and seven years of age, found that with the boys the order of preference was blue and yellow (both equal), then red, lastly green; while with the girls the order was green, blue, red and yellow; in combinations of two colors it was found that combinations of blue come first, then of yellow, then green, lastly red. It was found (as J. Cohn has found among adults and cultivated people) that the deepest and most saturated color was most pleasing; and also that the love of novelty and of variety was an important factor. It will be observed that at this age green was the girls’ favorite color and that least liked by the boys, whose favorite color, in combination, was blue; the number of individuals was, however, small. This was in Germany. In America, among 1,000 children, probably somewhat older on the average (though I have not details of the inquiry), Mr. Earl Barnes found, like Dr. Aars, that more boys than girls selected blue, while the girls preferred red more frequently than the boys; Barnes considers that with growing years there is a growing tendency to select red; as is well known, girls are more precocious than boys. Among 100 students at Columbia University, the order of preference was found to be blue (34 per cent), red (22.7 per cent), and then at a more considerable distance violet, yellow, green. It is noteworthy that among 100 women students at Wellesley College the order of preference was not very different, being blue (38 per cent), red (18 per cent), yellow, green, violet; in a later investigation the order remained the same, there being only some increase in the preference for red; it was considered that association accounted for the preference for blue, while more conscious as well as more emotional elements entered into the preference for red.
By far the most extensive investigation of color preference was that carried on at Chicago by Professor Jastrow on 4,500 persons, mostly adults, of both sexes and various nationalities.[H] Blue was found to be the favorite color, less than half as many persons preferring red; of every thirty men ten voted for blue and three for red, while of every thirty women five voted for red and four for blue. The men also liked violet and on the whole confined their choice to but few colors, the women also liked pink, green (very seldom chosen by men) and yellow, and showed a tendency to choose light and dainty shades. There was on the whole a decided preference for dark shades; the least favorite colors were yellow and orange. It is evident that, as we should expect, within the elementary field of popular æsthetics, women show a more trained feeling for color than men.
[H] J. Jastrow, “The Popular Æsthetics of Color,” Popular Science Monthly, 1897.
It is not quite easy to coördinate the various phenomena of color predilection. Careful and extended observations are still required. It seems to me, however, that the facts, as at present ascertained, do suggest a certain order and harmony in the phenomena. It is difficult not to believe that there really is, both among many uncivilized peoples and also many children at an early age, even to a slight extent among civilized adults, a relative inability, by no means usually absolute, to recognize and distinguish the tones of color at the more refrangible end of the spectrum. The earliest writers on the subject were wrong when they supposed that color nomenclature at all accurately corresponded to color perception, and it is well recognized that there are no peoples who are wholly unable to distinguish between green and blue and black. But as Garbini has clearly shown, there really is a parallelism between color nomenclature and color recognition, and Garbini’s wide investigation has confirmed the experiments of Preyer on a single child by showing that there is a certain hesitancy and uncertainty in recognizing the colors at the more refrangible end of the spectrum, long after children are familiar with the less refrangible end. In the same way the important investigations of Rivers have confirmed the earlier observations of Magnus and Almquist in showing that savages in many cases exhibit a certain difficulty in recognizing and distinguishing blue and green, such as they never experience with red and yellow. The vagueness of color nomenclature as regards blue and green thus indicates, though grossly exaggerating, a real psychological fact, and in this way we have an explanation of the curious fact that in widely separated parts of the world (at Torres Straits, among the Esthonians at Rome, etc.) as civilization progressed it was found necessary to borrow a word for blue from other languages.
There is almost complete harmony among a number of observers, now very considerable, in many countries, showing that the colors children first take notice of and recognize are red and yellow, most observers putting red first. There is no true predilection for these colors at this early age because the other colors do not yet seem to have been perceived. At first, doubtless, all colors appear to the infant as light or dark, white or black. That this is so is indicated by the experience of Dr. George Harley, who at one period of his life, in order to cure an injury to the retina caused by overwork at the microscope, resolutely spent nine months in absolutely total and uninterrupted darkness. When he emerged he found that, like an infant, he was unable to appreciate distance by the eye, while he had also lost the power of recognizing colors; for the first month all light colors appeared to him perfectly white and all dark colors perfectly black. He fails to state the order in which the colors reappeared to him. It is well recognized, however, that eyes long unexposed to light become color-blind for all colors except red. Preyer’s child in the fourth year was surprised that in the twilight her bright blue stockings looked grey, while for some time longer she always called dark green black. By the sixth year all colors are seen and known with fair correctness. Among young children at this age, so far as the evidence yet goes, red is rarely the preferred color, this being more often yellow, green or blue. There is doubtless room here for a great amount of individual difference, but on the whole it appears that children prefer those colors which they have most recently learnt to recognize, the colors which have all the charm of novelty and newly-won possession. It is probable, too, that (as Groos has also suggested) the stimulation of red is too painfully strong in this stage of the development of the color sense to be altogether pleasurable, in the same way that orchestral music is often only a disturbing noise to children.
One may note in this connection that hyperæsthesia to color is nearly always an undue sensibility to red and very rarely to any other color. The case has been recorded of a highly neurotic officer who, for more than thirty years, was intolerant of red-colored objects. The dazzling produced by scarlet uniforms, especially in bright sunshine, seriously interfered with the performance of his duties, and in private life red parasols, shawls, etc., produced similar effects; he was often overcome in the streets by giddiness, sometimes almost before he realized that he was looking at a red object. Many years ago Laycock referred to the case of a lady who could not bear to look at anything red, and Elliston also had a lady patient to whom red was very obnoxious, and who, when put into a room with red curtains, drank seven quarts of fluid a day. I am not aware that any such hyperæsthesia exists in the case of other colors. It is also noteworthy that the morbid affection in which color is seen when it does not exist is most usually a condition in which red is seen (erythropsia), yellow being the color most frequently seen after red (a condition called xanthopsia); the other colors are very rarely seen, and Hilbert, in his monograph on the pathology of the color sense, considers that this is due to the fact that red and yellow make the most intense effect on the sensorium, which thus becomes liable not only to direct but to reflected irritation, in the absence of any external color stimulus. There are other facts which show that of all colors red is that which acts as the most powerful stimulus on the organism. Münsterberg, in some interesting experiments which he made to illustrate the motor power of visual impressions as measured by their arresting action on the eye-muscles, found that red and yellow have considerably more motor power in stimulating the eye than the other colors. It may be added also that, as Quantz has found, we overestimate the magnitude of colors of the less refrangible part of the spectrum and underestimate the others.
After puberty blue seems still to maintain its position, but red has now come more to the front, while yellow has definitely receded; although so favorite a color in classic antiquity, it is rarely the preferred color among ourselves. J. Cohn in Germany found that among a dozen students it was never in any degree of saturation the preferred color, while at Cornell Major found that all the subjects investigated considered yellow and orange either unpleasant or among the least pleasant colors.
While blue seems to be the color most usually preferred by men, red is more commonly preferred by women, who also show a more marked predilection for its complementary green. Whether the feminine love of red shows a fine judgment we could better decide if we knew among what classes of the population red lovers and blue lovers respectively predominate; it may be noted, however, that the necessities of dress give the most ordinary woman an acquaintance with the elementary æsthetics of color which the average man has no occasion to acquire. In any case it might have been anticipated that, even though the typically ‘cold’ color should appeal most strongly to men, the most emotional of colors should appeal most strongly to women.