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say that it is a dark grayish or brownish field, full of irregular, confused, and ever-shifting lines and cloudings. If the retina has been previously strongly impressed, spectra are seen on this dark background when the eyes are shut. When the eyes are open, the same spectra are seen on the bright ground of the sky or wall, and the difference of the background makes the difference of the color of the spectra in the two cases.
Now the same inherent activity of the retina which produces the sense of a dark field with its confused markings and cloudings, will also, under certain circumstances of peculiar sensitiveness of the retina, as after complete rest in the early morning, give rise spontaneously to more definite spectra, often of beautiful colors. I have often, in bed in the morning, watched with eyes shut these splendid spectra, consisting of a colored patch surrounded with a border of complementary color, each color closing in on the center and so vanishing, while another border commences on the outside to close in in the same way. Thus, just as impressions or images made normally on the retina by actual objects from without are projected into the field of view and seen there as the true signs of objects, even so impressions made on the retina abnormally from within, by the mind or imagination, are also sometimes projected outward, and become the delusive signs of external objects having no existence. It is thus that the diseased brain gives rise to delusive visual phenomena.
Corresponding Points, Retinal and Spatial.-Further, it is evident that every point-every rod or cone-in the retina has its invariable correspondent in the visual field, and vice versa. Moreover, since the central
of the pencil of every radiant point in the external world passes through the nodal point of the crystalline lens, it is evident that these lines must cross each other there. In other words, the lines forming correspondent points in space and on the retina cross each other in the nodal point, and therefore the positions of these correspondent points, external and internal, are completely reversed. Thus not only are the retinal images inverted, but the relative positions of these images are inverted, and the position of every focal point is the inverse of its correspondent radiant point. It is obvious, then, that the left half of the retina corresponds with the right half of the field of view, and the right half of the former to the left half of the latter; and so also the upper half of the former corresponds to the lower half of the latter, and the lower half of the former to the upper half of the latter.
There are some peculiarities of vision which we are now prepared to explain.
1. 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 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 light. It differs again (6) by the presence of a pale-yellow coloring
matter in the retinal substance; hence it is sometimes called macula lutea—the yellow spot. It differs, again, (c) 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 58), only totoo 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. 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, or 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. 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.
It is evident, then, that distinctness of vision is a product of two factors, viz. : 1st, an optical apparatus for distinct image on the retina; and 2d, a retinal organization for distinct perception of the image thus formed. These two factors are perfectly independent of each other. If I hold up my pen before my eye, but very near, and then look at the sky, the outlines of the pen are blurred because the retinal image is so, but my perception is perfect. I can observe with great accuracy the exact degree of indistinctness. But if I hold the pen far to one side, say 90°, from the line of sight--on the extreme verge of the field of view—it is again indistinct, much more so than before, but from an entirely different cause, viz., imperfect perception of the retinal image. In fact, my perception is so imperfect that I can not tell whether the image is perfect or not. Thus there are two forms of indistinctness of vision, viz., indistinctness from imperfect retinal image, and indistinctness from imperfect retinal perception. The former is an effect of the optical instrument, the latter of the organization of the sensitive plate.
It is evident from the above that an elaborate structure of the lens, for making very exact images of objects on the margins of the field of view, would be of no use to man for want of corresponding distinctness of perception in the anterior margins of the retina. Therefore, as already stated on page 37, the peculiar structure of the crystalline, viz., its increasing density to the center, is of use to man only as correcting aberration, and not in conferring the faculty of periscopism. In the lower animals, however, in which periscopism is so important, this structure of the lens subserves both purposes. So far as this property is concerned, therefore, the structure in man may be regarded as having outlived its use.
Minimum Visibile.—Is there a limit to the smallness of a visible point? This question has been discussed by metaphysicians. But, as usually understood by them, there is no such thing as a minimum visibile. There is no point so small that it can not be seen if there be light enough. For example: a fixed star may be magnified 10 diameters, 100 diameters, 1,000 diam