Popular Science Monthly/Volume 40/March 1892/Wayside Optics
OUR train has been traveling for the past twenty-four hours over that part of a transatlantic route which stretches from the Sierra Madre to the extreme borders of the great Mohave Desert. There are many interesting things to be seen along this line of travel, but nothing more striking than the curious optical phenomena presented by the pleasing alternation of vast plain with rugged mountain. For example, not far from the last station we come upon a lofty peak overtopping the surrounding hills. It seemed to be about ten miles away, but was in reality fifty.
As is well known to the student of optics, the apparent size of an object is mainly dependent upon the size of the image which it makes upon the retina, just as its apparent distance from the observer is determined chiefly by the distinctness of the impression formed upon the background of the eye. The rays of light reflected from the distant mountain made a distinct image upon my retina) because they traversed a rarefied atmosphere of uniform density which produced the minimum amount of refraction, dispersion, and absorption. Previous to this time I had been accustomed, under Eastern skies, to view distant objects through media neither so rare nor so uniform as this mountain air, and it was not, therefore, strange that my calculations of distance should in this case be erroneous. Such phenomena, familiar enough to most travelers and to every dweller in the cool, thin atmosphere of mountainous regions, are almost startling when seen for the first time. It is difficult to believe that the huge, stony mass, apparently Fig. 1. so near—certainly so plainly seen—is over half a hundred miles away.
The illusion as to distance does not, however, extend to the matter of size. Mountains and hills may, under certain atmospheric conditions, appear to be near at hand when they are actually far away, but their apparent size remains always the same. The same mountain would appear of just the same size in Colorado as in Vermont. We know this because objects equally distant and of the same size always subtend the same visual angle. The greater the distance from the eye, the smaller the visual angle and retinal image; the less the distance, the greater the angle and the larger the image—as the following diagram (Fig. 1) shows:
The rays of light falling through the pupil upon the retina, b m c, cross at the nodal point a. The near object, 7 8, subtends a larger visual angle, 7 a 8, and makes a larger retinal image, 5 6, than the distant object, 1 2.
It would be interesting to test the truth of these statements by actual experiment, if ideas of size and distance did not, unfortunately, vary with the individual, and were not subject to almost daily modification by experience and other influences. Calculations as to the actual size and distance of the most familiar objects are, within certain limits, but pure guesses on the part of the great majority of people; so that, even if it were worth the while, the most of us could never become experienced enough, by making a study of distant objects, to do more than roughly approximate their actual size and distance away.
In making estimates of this kind we are, in the latter instance, very materially assisted by the peculiar "distance tints" which the mountains assume. The brain becomes accustomed, after a large number of experiences, to associate a certain coloration of objects with certain distances from the eye, and in this way to calculate the distance of an object seen for the first time. Einthoven thinks that the chromatic aberration which even a normal eye exhibits may account for the peculiar colored appearances which distant objects take on.
This explanation is manifestly opposed to the view commonly held, that the minute globules of water in the air act as prisms, and, resolving white light into its component colors, robe the distant mountains in "azure hues." In either case the peaks of the Sierras would deceive the unfamiliar eye, for not only are they more distinctly seen than their fellows of the Atlantic States, but their "distance tints" would entirely mislead the unaccustomed observer.
As the train proceeds rapidly over the level desert my eyes "fix"—i. e., gaze steadily at—a clump of sage-bush which is probably two miles distant. The bush seems to move slowly with the train, while objects between it and my eyes have an apparent motion in the opposite direction. Of these latter the near ones fly past with great rapidity, but the apparent velocity of those farther removed diminishes until, just before the point of fixation is reached, objects come to an apparent standstill. Beyond the point fixed by my eyes objects move in the same direction as the train, their velocity apparently greater the farther away they lie.
Suddenly I shift my gaze from the sage-bush to a large bowlder which is sailing slowly past, probably one thousand yards from the train. Everything is changed at once. The bowlder's retrograde progress is arrested; near objects fly past with accelerated speed; the sage-bush clump forges ahead as if to make up for lost time, while the plain beyond it, indistinct in the distance, races ahead of every object in view. And so I while away a full half-hour, making one conspicuous object after another stand still, go ahead, or sail past at will—all upon the surface of this apparently boundless plain—trying to realize, meantime, that things are not as the moving panorama before me indicates. For, relatively to the train, all objects are passed at an equal rate, the near as well as the distant, those seen by direct as well as those seen by indirect vision. But, in looking from my car window, I am made the subject of optical illusions common in a journey of this sort.
Notwithstanding the many wonderful things about the mechanism of vision, it exhibits, after all, a great many crudities. Intellectually, for instance, the optic centers are low down in the scale of origin. Even the olfactory nerves have a higher cerebral origin than they. Accordingly, we often find them committing all sorts of errors, from whose consequences only the experience of the other organs (acting as special detectives) enables the organism to escape.
Simple "seeing" ought not to be followed, in all cases, by implicit belief. When, for example, as in this case, the eye forms part of a moving mass, the motion is wrongly attributed by the optic centers to surrounding bodies. The explanation of how this comes about is easy when one considers certain facts in elementary optics. If I close one eye and slowly move a pen from right
to left a few inches in front of the other eye, the direction of the movement is rightly interpreted by my brain, although by a reference to Fig. 2 it will readily be seen that the retinal image of the pen moves in an opposite direction over the background of the eye.
Precisely the same effect is obtained if, instead of moving the pen, I look straight forward and move my head from left to right, simply because the same impression is produced—i. e., the retinal image moves from left to right.
When, therefore, the image of an object is made, it matters not how, to move over the retinal background, motion in an opposite direction is immediately referred to the object itself. It makes no difference, then, so far as the optical effect is concerned, whether the solid plain with the objects on its surface be carried past the observer at rest, or whether the observer himself move past or over the plain. Further, when there is no movement of the image over the retina, no motion is detected by the eye; optically, the object is at a standstill. That a body moving in front of the eye should appear to be stationary, its image must always be kept in the same position on the retina. This is accomplished by the alert ocular muscles. When, however, the object is too near the eye, or when its motion is too swift, the muscles are not quick enough in their action to preserve this delicate state of optical rest; the image is thrown across the retina, and the object is seen to move. A glance at this diagram (Fig. 3) will show how these retinal impressions are received and interpreted.
The first figure is intended to represent three objects seen from a train in motion. Although the middle one is fixed by the eye, Fig. 3. and is consequently most distinctly seen, the blurred images of the other two also fall upon the macula, so that for a single instant they are all optically at rest. A moment later, the eye, still fixing the middle object, has moved from 1 to 2, and, as is seen in 2, the images corresponding to the near and the remote objects have passed over the retinal area. Motion in the opposite direction is, according to the law just laid down, attributed to each, while the middle object still gives the impression of comparative rest.
When, however, the image of the moving object is kept fixed upon the macular region, the eye may judge of the rate of its motion by the amount of effort put forth by the ocular muscles necessary to keep the image focused upon the macula. This method of calculation is defective, and gives rise to numerous optical errors. For example, the movement of a lady's fan in front of her face, the velocity of a base-ball through the air five hundred yards off, and the rate at which the night express travels as it approaches "end on"—its head-light gleaming in the distance—would all be incorrectly calculated if the brain were to accept ocular evidence alone and based on one or both of the foregoing rules. The to-and-fro movement of the fan would be interpreted as exceedingly quick; the velocity of the base-ball would be next in order; while little or no motion would be attributed to the approaching train.
Becoming tired of looking at the wayside scenery, I find myself, in a sort of brown study, watching the back of the plush-covered seat in front of me, and then I discover that the retinal impressions made by the moving pageantry of the Arizona desert are curiously transferred to this crimson background. For I see a strip of plush moving irregularly to the right of me, and just above it another section moving to the left.
As the movements of the plush correspond very nearly to the previous visual impressions made by the moving landscape, I soon find that I can vary the plush movements at will.
Allowing sufficient intervals of rest to elapse, I am able to make an upper segment of the plush cushion move slowly backward or forward in contrast with a lower portion—a faithful photograph from the landscape negative.
This persistence of strong or continued retinal impressions may easily be demonstrated by another and commoner experiment. Look intently for two or three minutes at the light falling through a small window, other illumination being excluded. Then close the eyes and place a bandage over them. The impression produced by the light persists several minutes, and the experiment will be all the more striking if the window be crossed by bars, the persistent images of which are seen distinctly in strong contrast to the lighted spaces surrounding them.
Kühne, of Heidelberg, and others have shown that the retina possesses a pigmentary substance (visual purple), sensitive to light, which acts like the sensitized plate or film of the photographic camera, and that a picture distinctly seen is actually photographed upon the background of the eye.
Looking from the rear platform of our vestibule train—an admirable vantage-ground from which to view the country through which one is passing—I find that we have just skirted some foot-hills and are approaching the mouth of a small canon, at the head of which a bold, black mountain looks threateningly down on the desert below. The train once more gains the level country, and on looking back, although it is far up the gorge, the mountain seems very near. Nay, more, as I look first at the roadbed and then at the base of the huge mass in front of me, the latter, in some uncanny way, follows, as if it wished to fall upon and crush me. This apparent motion reminds me of Shelley's description:
"The Apennine in the light of day
Is a mighty mountain dim and gray,
Which between the earth and sky doth lay;
But when night comes, a chaos dread
On the dim starlight then is spread.
And the Apennine walks abroad with the storm."
Not so, however, is it with this particular outpost of the Sierra Madre. The fact is that while I have, at the car window, been experiencing the retinal effects produced by objects moving in a direction apparently parallel to the horizon, I am now having an object-lesson in optics with bodies whose apparent motion is at an angle to the horizontal line. In both these cases the explanation of the observed phenomena is precisely the same. The body of the mountain represents the most distant objects on the level desert, its base is the point of fixation, while the near objects are between it and the receding train.
The rails, sleepers, and the gravel fly past with a velocity which apparently diminishes in the distance; the mountain-base is practically at a standstill, but beyond it is the prominent bulk of the mountain itself, which appears to advance in obedience to the laws we have just been considering.
But my cerebral convolutions refuse to accept such evidence. They insist, these maturer products of the evolutionary force, that the organism has never had any experience of mountains chasing railway trains. And so it happens that I, placing my trust in an enlightened experience rather than in a report from my childish optical centers, feel assured that this particular mountain is not following us.
The prevailing idea that the organ of vision is practically a perfect piece of optical and nervous mechanism has done not a little to discourage attempts to develop those wonderful powers which it undoubtedly possesses.
One may, by judicious education, train and improve an undeveloped sense, but what improvement can be wrought in a perfected organism?
Far too little has been done in this direction, not only among children during school life, but in after-years spent at literary, technical, or other institutions. Of course, it may be asserted that the sense of sight, in conjunction with the other senses, receives its due share of developmental training in the ordinary course of general and special instruction. But, as opposed to this, may be urged, in the first place, the natural deficiencies of the eye, a few of which have just been referred to; and, in the second instance, the splendid results which, despite these innate defects, are obtainable by judicious training—results richer far than any other sense is capable of attaining.
This plea for a systematic exercise of the visual functions does not exclude the payment of proper attention to the other senses. It is asserted merely that our knowledge would be more complete if a larger proportion of the time and attention given to the cultivation of the special senses were devoted to the development of the capable but congenitally deficient organ of vision.
As a preliminary to this there should never be forgotten the care of sight. A great deal has been said (and too much, perhaps, can not be said) about the importance of ocular hygiene, especially as applied to schools and school children; but, in addition to these sanitary measures as applied to the mass, inspection of individual cases should be insisted upon. How many useful eyes might have been saved to the commonwealth if they had been examined and treated early in life by a competent oculist!
It is a rule—to which there are few exceptions—that, in addition to those defects which all eyes possess in common, the human organ of sight is, about the school age, prone to certain diseases, arising from inherent anomalies of structure, from heredity, from the results of infantile diseases, and from other causes. It is also true that many, if not most, of these dangers to which the eye in after-life is subject may be warded off by precautions suitable to individual cases. Thus the myope, or short-sighted person, should exercise care of a kind quite different from that which is suitable to the hyperope, or long-sighted individual; while the unfortunate astigmatic child (with "blurred" sight) should follow a prophylactic programme of a kind distinct from either; and so on through the list of possible ocular defects, which, although they commonly elude even the watchful eye of parent or guardian, are still possible sources of future disease. The advance of ophthalmological science has reached that point where one may read in the defective eyes of childhood the record of a large percentage of the impaired, restricted, or lost vision of later years.
- When the eye fixes anything, the visual apparatus is so adjusted that the rays of light coming from the object are focused upon the macula, a small central spot in the retina, where vision is most acute; and the object thus fixed is seen more distinctly than surrounding bodies.