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 1 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 per

ception 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

eters, 5,000 diameters, and still it is to us a mathematical point without dimensions. How much more, therefore, is it without dimensions to the naked eye! And yet it is perfectly visible. The only sense in which science recognizes a minimum visibile is the smallest space or object which can be seen as a surface or as a magnitude the smallest distance within which two points or two lines may approach each other and yet be perceived as two points or two lines. In this sense it is a legitimate inquiry; for there is here a real limit, which depends on the perfection of the eye as an instrument and the fineness of the organization of the retina.

We can best make this point clear by showing a similar property, but far less perfect, in the lower sense of touch. There is also a minimum tactile.

Experiment. Take a pair of dividers; stick on each point a mustard-seed shot, so that the impression on the skin shall not be too pungent. Now try, on another person whose eyes are shut, the least distance apart at which two distinct impressions can be perceived. It will be found that, on the middle of the back, it is about 3 inches; on the arm or back of the hand, it is about to inch; on the palm, about inch; on the finger-tips, about or inch; and on the tip of the tongue, about inch, or less.

Now, sight is a very refined tact, and the retina is specially organized for an extreme minimum tactile. There is no doubt that the size of the cones of the central spot determines the minimum visibile. If the images of two points fall on the same retinal cone, they will make but one impression, and therefore be seen as one; but if they are far enough apart to impress two cones, then they will be seen as two points. So also

of an object: if its image on the retina be sufficient to cover two or more cones of the central spot, then it will be seen as a magnitude. Taking the diameter of central-spot cones to be (which is the diameter given by some), the smallest distance between two points which ought to be visible at five inches distance is of an inch. This is found to be about

the fact in good eyes.

2. Blind Spot. This is the spot where the optic nerve enters the ball of the eye. Objects whose images fall on this spot are wholly invisible. It is for this reason that the point of entrance is always placed out of the axis, about inch on the nasal side. For, if it were in the axis, of course the image of the object we looked at would fall on this spot, and the object would consequently disappear from view. The structural cause of the blindness of this spot we have already explained on page 59. It is the absence of the bacillary layer. The existence of the blind spot may be easily proved by experiments which any one can repeat.

Experiment 1.-Make two conspicuous marks, A and B, a few inches apart. Then shut the left eye, and

while looking steadily with the right eye at the left object, A, bring the paper gradually nearer and nearer : at a certain point of approach B will disappear utterly. Continue to bring the paper nearer, still looking steadily at A at a certain nearer point B will reappear. The explanation is as follows: At first, when the paper is at considerable distance, say 18 inches, the image of A is, of course, on the central spot, for the axis of the eye is directed toward this point; but the image of B falls a little to the internal or nasal side of the central spot,