FIG. 28. bring the pin-head very near to the open eye, so that it touches the lashes, and in the line of sight: a perfect inverted image of the pin-head will be seen in the pinhole. If, instead of one, we make several pin-holes, an inverted image of the pin-head will be seen in each pin-hole, as shown in Fig. 28. The explanation is as follows: If the pin were farther away, say six inches or more, then light from the pin would be brought to focal points and produce an image on the retina; and this image, being inverted, would by projection be reinverted, and the pin would be seen in its real position. In the above experiment, however, the pin is much too near the retina to form a distinct image. But nearness to the retinal screen, though unfavorable for producing an image, is most favorable for casting a sharp shadow; and while retinal images are inverted, retinal shadows are erect. The true image of the pin, but very much blurred, may be dimly seen on the near side of the card and covering the pin-hole. The light streaming through the pin-hole into the eye casts an erect shadow of the pin-head on the retina. This shadow is projected outward into space, and by the law of direction is inverted in the act of projection, and therefore seen in this position in the pin-hole. It is further proved to be the outward projection of a retinal shadow by the fact that, by multiplying the pin-holes or sources of light, we multiply the shadows, precisely as shadows of an object in a room are multiplied by multiplying the lights in the room.* * This phenomenon was explained by the author in 1871. See "Philosophical Magazine," vol. lxi, p. 266. It had, however, been pre FIG. 29. a a Experiment 2.-If we look at a strong light, such as the flame of a candle or lamp, or a gas-flame, at some distance and at night, and then bring the lids somewhat near together, we observe long rays streaming from the light in many directions, but chiefly upward and downward. Fig. 29 gives the phenomenon as I see it. The explanation is as follows: In bringing the lids near together, the moisture which suffuses the eye forms a concave lens, as in Fig. 30 (hence the phenomenon is much more conspicuous if there be considerable moisture in the eyes). This watery lens will be saddle-shaped— i. e., concave vertically and convex horizontally. Now, the rays from the light (L, Fig. 29) which penetrate the center of the pupil will pass directly on without refraction except what is normal, and make its image (Fig. 30, L') on the central spot. But the rays which strike the curved surface of the watery lens will be bent upward to b and downward to a. Thus the light, instead of being brought to a focal point, is brought to a long focal line, ba, on the retina, with the image of the light in the middle at L'. The upper portion of this line b L' will be projected outward and downward, and form the downward streamers of Fig. 29; while the lower portion of the retinal impression a L' will be projected outward and upward, and form the upward streamers of Fig. 29. To prove this, while the streamers are conspicuous, with the finger lift up the upper lid: immediately the lower streamers viously explained by Priestley, but forgotten. ("Nature," vol. xxiv, p. 80, 1881.) 66 a disappear; now press down the lower lid: immediately FIG. 30. 6 nately one eye and the other, it will be seen that a b The much lighter diverging side-rays are more dif- 2. Properties of the Central Spot, and of its Representative in the Visual Field.-We have already stated that there are two spots on the retina where the constituent layers do not all exist. The central spot is destitute of nearly all except the bacillary layer; the blind spot, of all except the fibrous layer. The central spot (macula centralis) is a small depression not more than one thirtieth of an inch in diameter, situated directly in the axis of the eye, or what might be called the south pole of this globe. It differs from other parts of the retina (a) by wanting the fibrous and granular layers; therefore the retina is much thinner there, and the spot is consequently pit-shaped, and on this account is often called the fovea centralis, or central pit. Of course, the absence of other layers exposes the bacillary layer here to the direct action of L a light. It differs again (b) by the presence of a paleyellow coloring matter in the retinal substance; hence it is sometimes called macula lutea-the yellow spot. It differs, again, (e) in a finer organization than any other part of the retina. The bacillary layer here consists only of cones, and these are far smaller, and therefore more numerous, than elsewhere; being here, as already seen (page 55), only obo to Too of an inch in diameter. Function of the Central Spot.-Every point on the retina, as already seen, has its correspondent or representative in the field of view. Now, what is the representative of the central spot? It is evidently the point, or rather the line, of sight, and a small space immediately about it. From its position in the axis of the eye, it is evident that on it must fall the image of the object or part of the object looked at, and of all points in the visual line or line of sight. Now, if we look steadily and attentively on any spot on the wall, and, without moving the eyes, observe the gradation of distinctness over the field, we find that the distinctness is most perfect at the point of sight and a very small area about that point, and becomes less and less as we pass outward in any direction toward the margins of the field of view. Standing two feet from the wall, I look at my pen held at arm's length against the wall, and of course see the pen distinctly. Looking still at the same spot, I move the pen to one side eight or ten inches: I now no longer see the hole in the back of the pen. I move it two feet or more to one side: I now no longer see the shape of the pen. I see an elongated object of some kind, but can not recognize it as a pen without turning my eyes and bringing its image on the central spot. Hence, to see distinctly a wide field, as in looking at a landscape or a picture, we unconsciously and rapidly sweep the line of sight over every part, and then gather up the combined impression in the memory. To read a printed page we must run the eye from word to word, so that the image of each in succession shall fall on the central spot. Now, the point of sight with a very small area about it corresponds to the central spot, and the margins of the field of view correspond to the extreme forward margin of the retina. Therefore the organization of the retina for distinct perception is most perfect in the central spot, and becomes gradually less and less perfect as we pass toward the anterior margin, where its perception is so imperfect that we can not tell exactly where the field of view ends, except where it is limited by some portion of the face. Now, what is the use of this arrangement? Why would it not be much better to see equally distinctly over all portions of the field of view? I believe that the existence of the central spot is necessary to fixed, thoughtful attention, and this again in its turn is necessary for the development of the higher faculties of the mind. In passing down the animal scale, the central spot is quickly lost. It exists only in man and the higher monkeys. In the lower animals, it is necessary for safety that they should see well over a very wide field. In man, on the contrary, it is much more necessary that he should be able to fix undivided attention on the thing looked at. This would obviously be impossible if other things were seen with equal distinctness. This subject is more fully treated in the final chapter of this work.* * A central spot, though differing from that of man, is found also in some birds." American Naturalist," vol. xxx, p. 24, 1896. |