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It is evident, then, that distinctness of vision is a product of two factors, viz.: first, an optical apparatus for distinct image on the retina ; and, second, 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 external image is blurred 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 31, 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.

The central spot is certainly the most sensitive and highly organized part of the retina. We can not see accurately unless the image falls on this spot. And yet it is a curious fact that other parts of the retina are more sensitive to mere light as light irrespective of form and color. In very faint light the mere presence of an object may be detected by indirect vision when it can not be detected by direct vision. It is well known that a faint star may be seen by looking a little to one side, when it can not be seen if looked at directly. The same is true of any very faint object at night.

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 diameters, 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 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 7 inch; on the fingertips, about 1 or 1 inch; and on the tip of the tongue, about z 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 centralspot cones to be mobo (which is the diameter given by some), the smallest distance between two points which ought to be visible at five inches distance is Toto of an inch. This is found to be about the fact in good eyes.

3. Blind Spot and its Representative in the field of View.—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 57. It is the absence of the bacillary layer; and this absence is the necessary result of the turning back of the fibers of the optic to terminate in the bacillary layer. As we shall see hereafter (page 308), the blind spot is peculiar to the vertebrate eye. 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, in this case about 7 inches, 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, viz., between the central spot and the blind spot. Now, as the paper comes nearer,

turns more and more in order to regard A, the image of B travels slowly over the retina noseward until it reaches the blind spot, and the object disappears. As the paper still approaches, the image of B continues to travel in the same direction until it crosses over the blind spot to the other side, when the object immediately reappears.

the eye

FIG. 31.






The accompanying diagram, Fig. 31, illustrates this phenomenon. Let A and B represent the two objects, and R and I the positions of the right and left eyes respectively. The right is drawn, but the left, being shut, is not drawn, but only its position indicated by the dot. The central spot is represented by c, in the axis A c, and the blind spot by o, where the optic nerve enters. It is obvious that the image a of the object A will be La always on c, and the place of the image of B is on the intersection b of the line B b with Ly the retina. Now, as the eye approaches the objects A and B, it is seen that the image b of B travels toward the blind spot, o. At the second position of the eye, R', it has not reached it. At the third position, R", it is upon it.

At the fourth position, R''', it has already crossed over and is now on the other

R side. At the third position, R”, the object B disappears from view. The distance at which the disappearance takes place will, of course, depend on the distance between the objects A and B. If these are 3 inches apart, then the disappearance on approach from a greater distance takes place at about 1 foot, and the reappearance at about 10 inches. If



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