Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

Fig. 246.

354. Distinct Vision.—In order that vision may be perfectly distinct, it is necessary that the rays coming from each point of the object which is seen should, on converging, meet together, or be brought to a focus on the screen of the eye, the retina. Thus, in Fig. 245, the rays which come from a, the end of the arrow, meet on the retina at b, and those from c, the other end, are brought to a focus at d. Now the muscles of the eye have considerable power in adjusting the eye to objects at different distances, so as to bring the rays in most cases together exactly at the retina. They fail to do it with objects that are very near. You can see that this is so if you bring any object, as your finger, nearer and nearer to the eye. You will at length find that you can not see it distinctly. The reason is, that the rays from it diverge so much that the cornea and lens can not make them converge enough to meet at the retina. This divergence of rays at different distances is illustrated in Fig. 246. Suppose that you are looking at some very minute object. The nearer you bring it to the eye the better you can see it, till you come to a certain point. There the rays are so divergent, as you can readily see by the figure, that the lenses of the eye can not make them converge sufficiently for distinct vision. Now just here the microscope comes in to help the eye by causing these divergent rays to come nearer together before they enter the window of the eye, the cornea.

Fig. 247.

Fig. 248.

355. Near-Sighted and Far-Sighted.—Some persons have their eyes so shaped that they can not fully adjust them to objects at different distances. Thus the near-sighted can see with distinctness only objects that are near. The reason is that the rays converge too much, and are brought to a focus before they arrive at the retina, as represented in Fig. 247. The images therefore of distant objects are indistinct. If the retina could in any way be brought forward a little the difficulty would be obviated. But as this can not be done, concave glasses are resorted to, which counteract the effect of the too highly refractive power of the eye. In the far-sighted the difficulty is of an opposite character. The refractive power is so feeble that when near objects are viewed the rays are not brought to a focus soon enough, as seen in Fig. 248. Convex glasses are used in this case, making the divergent rays of near objects less divergent before they enter the cornea.

356. Images in the Eye Inverted.—The images formed on the retina are inverted. This can be proved by taking the eye of an ox and carefully paring off the back of it, leaving little else than the retina itself. Holding now a candle before the eye, its image may be seen inverted upon its rear part. The question arises why it is that we see objects erect when their images on the retina are inverted. On this point I will quote from my Human Physiology: "It has been supposed by some that we really see every thing reversed, and that our experience with the sense of touch, in connection with that of vision, sets us right in this particular. And this it is supposed is the more readily done from the fact that our own limbs and bodies are reversed as pictured on the retina, as well as objects that are around us, so that every thing is relatively right in position. But if this be the true explanation, those who have their sight restored after having been blind from birth should at first see every thing wrong side up, and should be conscious of rectifying the error by looking at their own limbs and bodies. But this is not so. The above explanation of erect vision, and other explanations of a similar character, are based upon a wrong idea of the office which the nerve performs in the process of vision. It is not the image formed upon the retina which is transmitted to the brain, but an impression produced by that image. The mind does not look in upon the eye and see the image, but it receives an impression from it through the nerve; and this impression is so managed that the mind gets the right idea of the relative position of objects. Of the way in which this is done we know as little as we know of the nature of the impression itself."

357. Single Vision.—Whenever we see any object with both eyes there is an image formed in each eye, and impressions go from both eyes by the optic nerves to the brain. And yet with these two impressions there is no double vision so long as the two eyes correspond with each other in situation. This is because the image in one eye occupies the same place on the retina that the image in the other eye does. The correspondence is ordinarily perfect, the two eyes turning always together in the same way, upward, downward, or laterally, without the least variation. You can observe the effect of a want of this correspondence by pressing one of the eyes in some direction with the finger while the other is left free to move in obedience to the muscles. When this is done every object appears double, because its image occupies in one eye a different part of the retina from what it does in the other, and so two different impressions are carried to the brain. The same thing occurs in squinting, in which the action of the muscles of the two eyes does not agree. Ordinarily in squinting there is not double vision, because the mind has the habit of disregarding the impressions that come from the defective eye. But when squinting occurs suddenly from disease there is double vision, for it takes a little time to form the habit referred to.

358. Stereoscope.—The images of objects in the two eyes, though always similar, are not generally perfectly alike. They are so only when the object presents a simple surface, as in the case of pictures. When the object presents two or more surfaces to the sight the images are more or less unlike. This can be illustrated in a very simple way. Hold a book up straight before your eyes with its back toward you. You see the back and both sides. Now if you shut your right eye you will see with the left the back of the book and the left side. That is, these two parts of the book are imaged on the retina of the left eye. By shutting the left eye it will appear that the image in the right is different, for you see now with the back the right side of the book. Here you have the explanation of the stereoscope. In the right side of this instrument you have the picture of the object as the object itself would appear to the right eye, and in the left side you have the picture of it as it would appear to the left eye. Thus, if a book in the position alluded to above were the object, in the right picture there should be represented the back together with the right side of the cover, and in the left the back with the left side of the cover. The two impressions, carried to the brain by the optic nerves, give together the impression of a solid book. The same principles apply to the representation of all solids in the stereoscope.