SUNLIGHT AND SKYLIGHT AT HIGH ALTITUDES.
At the Southampton meeting of the British Association, Captain Abney read a paper in which he called attention to the fact that photographs taken at high altitudes show skies that are nearly black by comparison with bright objects projected against them, and he went on to show that the higher above the sea level the observer went, the darker the sky really is and the fainter the spectrum. In fact, the latter shows but little more than a band in the violet and ultraviolet at a height of 8,500 feet, while at sea-level it shows nearly the whole photographic spectrum. The only reason of this must be particles of some reflecting matter from which sunlight is reflected. The author refers this to watery stuff, of which nine-tenths is left behind at the altitude at which be worked. He then showed that the brightness of the ultra-violet of direct sunlight increased enormously the higher the observer went, but only to a certain point, for the spectrum suddenly terminated about 2,940 wave-length. This abrupt absorption was due to extra-atmospheric causes and perhaps to space. The increase in brightness of the ultra-violet was such that the usually invisible rays, L, M, N, could be distinctly seen, showing that the visibility of these rays depended on the intensity of the radiation. The red and ultra-red part of the spectrum was also considered. He showed that the absorption lines were present in undiminished force and number at this high altitude, thus placing their origin to extra-atmospheric causes. The absorption from atmospheric causes of radiant enemy in these parts he showed was due to "water-stuff," which he hesitated to call aqueous vapor, since the banded spectrum of water was present, and not lines. The B and A line he also stated could not be claimed as telluric lines, much less as due to aqueous vapor, but must originate between the sun and our atmosphere. The author finally confirmed the presence of benzine and ethyl in the same region. He had found their presence indicated in the spectrum at sea-level, and found their absorption lines with undiminished intensity at 8,500 feet. Thus, without much doubt, hydrocarbons must exist between our atmosphere and the sun, and, it may be, in space.
Prof. Langley, following Capt. Abney, observed: The very remarkable paper just read by Captain Abney has already brought information upon some points which the one I am about, by the courtesy of the Association, to present, leaves in doubt. It will be understood then that the references here are to his published memoirs only, and not to what we have just heard.
The solar spectrum is so commonly composed to have been mapped with completeness, that the statement that much more than one-half its extent is not only unmapped but nearly unknown, may excite surprise. This statement is, however, I think, quite within the truth, as to that almost unexplored region discovered by the elder Herschel, which, lying below the red and invisible to the eye, is so compressed by the prism that, though its aggregate heat effects have been studied through the thermopile, it is only by the recent researches of Capt. Abney that we have any certain knowledge of the lines of absorption there, even in part. Though the last-named investigator has extended our knowledge of it to a point much beyond the lowest visible ray, there yet remains a still remoter region, more extensive than the whole visible spectrum, the study of which has been entered on at Alleghany, by means of the linear bolometer.
The whole spectrum, visible and invisible, is powerfully affected by the selective absorption of our atmosphere and that of the sun; and we must first observe that could we get outside our earth's atmospheric shell, we should see a second and very different spectrum, and could we afterward remove the solar atmosphere also, we should have yet a third, different from either. The charts exhibited show:
1st. The distribution of the solar energy as we receive it, at the earth's surface, throughout the entire invisible as well as visible portion, both on the prismatic and normal scales. This is what I have principally to speak of now, but this whole first research is but incidental to others upon the spectra before any absorption, which though incomplete, I wish to briefly allude to later. The other curves then indicate:
2d. The distribution of energy before absorption by our own atmosphere.
3d. This distribution at the photosphere of the sun. The extent of the field, newly studied, is shown by this drawing [chart exhibited]. Between H in the extreme violet, and A in the furthest red, lies the visible spectrum, with which we are familiar, its length being about 4,000 of Angstrom's units. If, then, 4,000 represent the length of the visible spectrum, the chart shows that the region below extends through 24,000 more, and so much of this as lies below wave-length 12,000, I think, is now mapped for the first time.
[Illustration: FIG. 1.--PRISMATIC SPECTRUM.]
We have to pi = 12,000 relatively complete photographs, published by Capt. Abney, but, except some very slight indications by Lamansky, Desains, and Mouton, no further guide.
Deviations being proportionate to abscissae, and measured solar energies to ordinates, we have here (1) the distribution of energy in the prismatic, and (2) its distribution in the normal spectrum. The total energy is in each case proportionate to the area of the curve (the two very dissimilar curves inclosing the same area), and on each, if the total energy be roughly divided into four parts, one of these will correspond to the visible, and three to the invisible or ultra-red part. The total energy at the ultra violet end is so small, then, as to be here altogether negligible.
We observe that (owing to the distortion introduced by the prism) the maximum ordinate representing the heat in the prismatic spectrum is, as observed by Tyndall, below the red, while upon the normal scale this maximum ordinate is found in the orange.
I would next ask your attention to the fact that in either spectrum, below pi = 12,000 are most extraordinary depressions and interruptions of the energy, to which, as will be seen, the visible spectrum offers no parallel. As to the agent producing these great gaps, which so strikingly interrupt the continuity of the curve, and, as you see, in one place, cut it completely into two, I have as yet obtained no conclusive evidence. Knowing the great absorption of water vapor in this lowest region, as we already do, from the observations of Tyndall, it would, a priori, seem not unreasonable to look to it as the cause. On the other hand, when I have continued observations from noon to sunset, making successive measures of each ordinate, as the sinking sun sent its rays through greater depths of absorbing atmosphere, I have not found these gaps increasing as much as they apparently should, if due to a terrestrial cause, and so far as this evidence goes, they might be rather thought to be solar. But my own means of investigation are not so well adapted to decide this important point as those of photography, to which we may yet be indebted for our final conclusion.
[Illustration: FIG. 2.--NORMAL SPECTRUM. (At sea level.)]
I am led, from a study of Capt. Abney's photographs of the region between pi = 8,000 and pi = 12,000, to think that these gaps are produced by the aggregation of finer lines, which can best be discriminated by the camera, an instrument which, where it can be used at all, is far more sensitive than the bolometer; while the latter, I think, has on the other hand some advantage in affording direct and trustworthy measures of the amount of energy inhering in each ray.
One reason why the extent of this great region has been so singularly underestimated, is the deceptively small space into which it appears to be compressed by the distortion of the prism. To discriminate between these crowded rays, I have been driven to the invention of a special instrument. The bolometer, which I have here, is an instrument depending upon principles which I need not explain at length, since all present may be presumed to be familiar with the success which has before attended their application in another field in the hands of the President of this Association.
I may remark, however, that this special construction has involved very considerable difficulties and long labor. For the instrument here shown, platinum has been rolled by Messrs. Tiffany, of New York, into sheets, which, as determined by the kindness of Professor Rood, reach the surprising tenuity of less than one twenty-five-thousandth of an English inch (I have also iron rolled to one fifteen-thousandth inch), and from this platinum a strip is cut one one-hundred-and-twenty-fifth of an inch wide. This minute strip, forming one arm of a Wheatstone's bridge, and thus perfectly shielded from air currents, is accurately centered by means of a compound microscope in this truly turned cylinder, and the cylinder itself is exactly directed by the arms of this Y.
The attached galvanometer responds readily to changes of temperature, of much less than one-ten-thousandth degree F. Since it is one and the same solar energy whose manifestations we call "light" or "heat," according to the medium which interprets them, what is "light" to the eye is "heat" to the bolometer, and what is seen as a dark line by the eye is felt as a cold line by the sentient instrument. Accordingly, if lines analogous to the dark "Fraunhofer lines" exist in this invisible region, they will appear (if I may so speak) to the bolometer as cold bands, and this hair-like strip of platina is moved along in the invisible part of the spectrum till the galvanometer indicates the all but infinitesimal change of temperature caused by its contact with such a "cold band." The whole work, it will be seen, is necessarily very slow; it is in fact a long groping in the dark, and it demands extreme patience. A portion of its results are now before you.
The most tedious part of the whole process has been the determination of the wave-lengths. It will be remembered that we have (except through the work of Capt. Abney already cited, and perhaps of M. Mouton) no direct knowledge of the wave-lengths in the infra-red prismatic spectrum, but have hitherto inferred them from formulas like the well-known one of Cauchy's, all which known to me appear to be here found erroneous by the test of direct experiment, at least in the case of the prism actually employed.
I have been greatly aided in this part of the work by the remarkable concave gratings lately constructed by Prof. Rowland, of Baltimore, one of which I have the pleasure of showing you. [Instrument exhibited.]
The spectra formed by this fall upon a screen in which is a fine slit, only permitting nearly homogeneous rays to pass, and these, which may contain the rays of as many as four overlapping spectra, are next passed through a rock-salt or glass prism placed with its refracting edge parallel to the grating lines. This sorts out the different narrow spectral images, without danger of overlapping, and after their passage through the prism we find them again, and fix their position by means of the bolometer, which for this purpose is attached to a special kind of spectrometer, where its platinum thread replaces the reticule of the ordinary telescope. This is very difficult work, especially in the lowermost spectrum, where I have spent over two weeks of consecutive labor in fixing a single wave-length.
The final result is, I think, worth, the trouble, however, for, as you see here, we are now able to fix with approximate precision and by direct experiment, the wave-length of every prismatic spectral ray. The terminal ray of the solar spectrum, whose presence has been certainly felt by the bolometer, has a wave-length of about 28,000 (or is nearly two octaves below the "great A" of Fraunhofer).
So far, it appears only that we have been measuring heat, but I have called the curve that of solar "energy," because by a series of independent investigations, not here given, the selective absorption of the silver, the speculum-metal, the glass, and the lamp-black (the latter used on the bolometer-strip), forming the agents of investigation, has been separately allowed for. My study of lamp-black absorption, I should add in qualification, is not quite complete. I have found it quite transparent to certain infra-red rays, and it is very possible that there may be some faint radiations yet to be discovered even below those here indicated.
In view of the increased attention that is doubtless soon to be given to this most interesting but strangely neglected region, and which by photography and other methods is certain to be fully mapped hereafter, I can but consider this present work less as a survey than as a sketch of this great new field, and it is as such only that I here present it.
All that has preceded is subordinate to the main research, on which I have occupied the past two years at Alleghany, in comparing the spectra of the sun at high and low altitudes, but which I must here touch upon briefly. By the generosity of a friend of the Alleghany Observatory, and by the aid of Gen. Hazen, Chief Signal Officer of the U S. Army, I was enabled last year to organize an expedition to Mount Whitney in South California, where the most important of these latter observations were repeated at an altitude of 13,000 feet. Upon my return I made a special investigation upon the selective absorption of the sun's atmosphere, with results which I can now only allude to.
By such observations, but by methods too elaborate for present description, we can pass from the curve of energy actually observed to that which would be seen if the observer were stationed wholly above the earth's atmosphere, and freed from the effect of its absorption.
The salient and remarkable result is the growth of the blue end of the spectrum, and I would remark that, while it has been long known from the researches of Lockyer, Crova, and others that certain rays of short wave-length were more absorbed than those of long, these charts show how much separate each ray of the spectrum has grown, and bring, what seems to me, conclusive evidence of the shifting of the point of maximum energy without the atmosphere toward the blue. Contrary to the accepted belief, it appears here also that the absorption on the whole grows less and less, to the extreme infra-red extremity; and on the other hand, that the energy before absorption was so enormously greater in the blue and violet, that the sun must have a decidedly bluish tint to the naked eye, if we could rise above the earth's atmosphere to view it.
But even were we placed outside the earth's atmosphere, that surrounding the sun itself would still remain, and exert absorption. By special methods, not here detailed, we have at Alleghany compared the absorption, at various depths, of the sun's own atmosphere for each spectral ray, and are hence enabled to show, with approximate truth, I think for the first time, the original distribution of energy throughout the visible and invisible spectrum at the fount of that energy, in the sun itself. There is a surprising similarity, you will notice, in the character of the solar and telluric absorptions, and one which we could hardly have anticipated a priori.
Here, too, violet has been absorbed enormously more than the green, and the green than the red, and so on, the difference being so great, that if we were to calculate the thickness of the solar atmosphere on the hypothesis of a uniform transmission, we should obtain a very thick atmosphere from the rate of absorption in the infra-red alone, and a very thin one from that in the violet alone.
But the main result seems to be still this, that as we have seen in the earth's atmosphere, so we see in the sun's, an enormous and progressive increase of the energy toward the shorter wave-lengths. This conclusion, which, I may be permitted to remark, I anticipated in a communication published in the Comptes Rendus of the Institute of France as long since as 1875, is now fully confirmed, and I may mention that it is so also by direct photometric methods, not here given.
If, then, we ask how the solar photosphere would appear to the eye, could we see it without absorption, these figures appear to show conclusively that it would be blue. Not to rely on any assumption, however, we have, by various methods at Allegheny, reproduced this color.
Thus (to indicate roughly the principle used), taking three Maxwell's disks, a red, green, and blue, so as to reproduce white, we note the three corresponding ordinates at the earth's surface spectrum, and, comparing these with the same ordinates in the curve giving the energy at the solar surface, we rearrange the disks, so as to give the proportion of red, green, and blue which would be seen there, and obtain by their revolution a tint which must approximately represent that at the photosphere, and which is most similar to that of a blue near Fraunhofer's "F."
The conclusion, then, is that, while all radiations emanate from the solar surface, including red and infra-red, in greater degree than we receive them, the blue end is so enormously greater in proportion that the proper color of the sun, as seen at the photosphere is blue--not only "bluish," but positively and distinctly blue; a statement which I have not ventured to make from any conjecture, or on any less cause than on the sole ground of long continued experiments, which, commenced some seven years since, have within the past two years irresistibly tended to the present conclusion.
The mass of observations on which it rests must be reserved for more detailed publication elsewhere. At present, I can only thank the association for the courtesy which has given me the much prized opportunity of laying before them this indication of methods and results.