By the aid of an instrument called a spherometer, which substitutes the sense of touch for that of sight, an inch may be divided into twenty thousand parts; and the lever of contact, an instrument in use among the German opticians, enables them to appreciate quantities of space even yet smaller. [Footnote 52] Instruments have been devised capable of measuring intervals of time equal to the 1/1000 part of a second. By the revolution of a toothed wheel, striking against a piece of card, human ear is enabled to appreciate a sound which lasts only 1/24000 of a second, and thus to measure that extremely minute interval of time. [Footnote 53] Wheatstone, in the course of his experiments on the velocity of the electric fluid, constructed an apparatus which enables the eye to perceive an interval equal to less than 1/1000000 of a second of time. The exact value of this almost infinitesimal interval was ascertained and measured by the known effect of a sound of high high pitch upon the ear. [Footnote 54] It is unnecessary to multiply such examples; but so many we have adduced, for the purpose of demonstrating the extent of the world of physical observation which lies forever concealed from the natural organs of sense. We owe this knowledge of their incapacity for more than a very limited range of observation to the inventions of science, applied to remedy and supplement this very incapacity. Thus science tells tales against the human senses, of which a less inventive and informed age could never have even dreamed.
[Footnote 52: Herschel's Discourse on the Study of Natural Philosophy, § 338.]
[Footnote 53 Somerville's Connexion, etc., § xvi. p. 147.]
[Footnote 54: Ib., § xxviii. p. 325.]
Once more, (5.) the senses are not only restricted in their sphere of action, and incapable of penetrating beyond a certain limit into the mysteries of physical nature, but even within their own proper province of observation their indications are constantly false and erroneous; so that if we were implicitly to receive and adopt these indications, without due correction, our notions of the constitution of nature would be singularly wide of the truth. As they appear to the naked eye, the sun and moon seem nearly of the same size; flat discs, about as large as the crown of a hat. Uncorrected sense teaches us no more; it furnishes no means of measuring either their absolute or their relative distance. But from other sources, we learn that one is about four hundred times further off than the other; that the mass of the one would fill a space bounded by double the orbit of the other; and that the centre of the sun is nearly half a million of miles nearer our eye than his limb, or the bounding line of his disc, a space equal to more than twice the distance of the moon from the earth. The limits prescribed to himself, forbid the author to enlarge on this interesting portion of his subject, which, however, he regrets the less, that any one anxious to follow it out, will find an excellent paper on "Popular Fallacies," in Lardner's Museum of Science and Art, January 1854; a new scientific and popular serial, which has started under the best auspices, and deserves to be widely circulated.
Did space permit, we might illustrate the fallacious teaching of the senses regarding the phenomena of nature, by the corrections made necessary in every scientific observation, as to the position of distant objects, in consequence of the refraction or bending of the rays of light in their passage through the air, which has the effect of making distant objects in space seem higher than they really are; of the correction necessary for the aberration of light, depending on the time taken to transmit it from a distant object in space; together with others which enter into the daily experience of the observers of nature. Other circumstances also materially influence the impressions conveyed through the organs of sense. Thus a person going into an ordinarily lighted apartment from the dark night, will be painfully affected by the brightness of the light [{262}] for a few moments; while another, entering the same room from a brightly illuminated chamber, will hardly be able for a moment or two to see anything. [Footnote 55] If we plunge our hands one into ice-cold water, and the other into water as hot as it can be borne, and after letting them stay a while, suddenly transfer them both to a vessel full of water at blood heat, the one will feel it hot, and the other cold. If we cross the two first fingers of our hand, and place a pea in the fork between them, moving and rolling it about on a table, we shall be fully persuaded, especially if we close our eyes, that we have two peas. [Footnote 56] The other senses are similarly affected by circumstances, so as to convey erroneous impressions. Mrs. Somerville sums up the evidence on this head in one word, when she remarks that, "a consciousness of the fallacy of our senses is one of the most important consequences of the study of nature. This study teaches us that no object is seen by us in its true place." [Footnote 57] And elsewhere she adds, "A high degree of scientific knowledge has been necessary to dispel the errors of the senses ." [Footnote 58] Herschel has the following remark in his Outlines of Astronomy: [Footnote 59] "No geometrical figure, or curve, is seen by the eye as it is conceived by the mind to exist in reality. The laws of perspective interfere and alter the apparent directions, and foreshorten the dimensions of its several parts. If the spectator be unfavorably situated, as, for instance, nearly in the plane of the figure, they may do so to such an extent as to make a considerable effort of imagination necessary to pass from the sensible to the real form."
[Footnote 55: Carpenter's Manual of Physiology, § 93.]
[Footnote 56: Herschel's Discourse, § 72.]
[Footnote 57: Collection of Physical Sciences, § xxv. p. 264.]
[Footnote 58: Ib., § iv. p. 37.]
[Footnote 59: Chap. i. §78.]
There is one form of illusion to which the senses are liable, so remarkable and irremediable as to deserve a moment's notice; we mean their erroneous testimony regarding motion. We have the authority of Sir. J. Herschel for saying, that "there is no peculiar sensation which advertises us that we are in motion. The rough inequalities in the road are felt as we are carried over them, by the successive elevation and falling of the carriage; but we have no sense of progress if we are prevented from seeing surrounding objects. The smoother the road, and the faster the speed, the less able are we to feel our motion forward. Every one must have felt this in night travelling by the railway, or in a tunnel. In a balloon, with a steady breeze, which merely propels, without gyration or oscillation, the motion is described as a sensation of perfect rest. The same is observed on shipboard, in still water or a calm. Everything goes on as if on land." [Footnote 60] To complete the illusion, nothing is more common than apparently to transfer our own motion to the stationary objects around us. This is peculiarly observable at railway stations, when a train first gently moves off. If another training is standing near, and parallel to our own, it is impossible to tell which is moving, our own, or the other in an opposite direction, without calling in the age of a third object, to correct the doubtful or erroneous impression, by the direction in which it seems relatively to change its place; or by examining the wheels of the other training. In the same way, many persons, while witnessing a panorama, are painfully affected by the shifting of the scenes, which conveys to them an impression as if the room were going round, and the picture remaining stationary. It was this illusion of the senses, as to motion, that perpetuated to a very late date the capital error regarding the supposed circulation of the sun and planets round the on moving; the dispelling of which, by Galileo and subsequent observers, was the greatest triumph ever achieved my philosophy over the empire of the senses.
[Footnote 60: Outline of Astronomy, § 15, 16.]
The simple matter of fact is this, that our senses were given us for a certain definite and practical end, not for the acquisition of universal knowledge. We use them thankfully within their own domain, but we should err by inferring that their indications are the measure of the true, or of the whole constitution of things: their teaching falls far short of what exists in the universe of material nature; into the world of spiritual existence and operation they have no mission to enter. Catholic doctrine, therefore, is in no worse position, as regards the contradictions of the senses to the results, than is the great mass of scientific knowledge; to deny the one is as unphilosophical as to deny the other, merely because the organs of sense fail to appreciate it, or afford indications directly contrary to it.