Further, it will be seen that these layers exist, all three, in every part of the retina except two spots. These are the spots where the optic nerve, o, enters, and the central spot, c, which is in the axis of the eye. Where the optic nerve enters, of course, no other layer can exist except the fibrous layer. In the central spot the fibrous layer is wholly wanting, and the granular and nuclear layer is almost wanting, so that the retina is here almost reduced to the bacillary layer. For this reason this spot forms a depression in the retina.

But the extreme importance of the retina requires that these layers be examined more closely. For this a much greater enlargement is necessary. Fig. 22 represents such enlargement. The fibrous layer, h, requires no further description; but the granular and nuclear layer is seen to be composed of two distinct layers of small granules, d and ƒ, and two layers of large nucleated cells, c and e, and a layer of very large nucleolated cells, g, from which go out branching fibers. These are multipolar cells, or ganglia. It is further seen that the bacillary layer is composed of two kinds of elements, viz., slender cylindrical rods and larger cone-like bodies. These are called rods and cones. It is seen, still further, that all these different elements of the retina are in continuous connection with each other, and with the fibers. of the optic nerve.

The bacillary layer is of the extremest interest. It consists mostly of rods, but among these are distributed the larger cones, as in Fig. 23, A. As we approach the central spot the cones become more numerous, as seen in B. In the depression of the central spot (fovea centralis) we find only cones, and these are of much smaller size than those in other parts of the retina, as seen in C. The rods are about inch in length and

350

14000 inch in diameter. The cones are shorter and about three times thicker than the rods, except in the central depression, where they are nearly as small as the rods, being there only 1 inch in diameter. In this 10000 spot, therefore, there are probably no less than one million cones in a square inch.

A

FIG. 23,

BACILLARY LAYER, VIEWED FROM THE OUTSIDE SURFACE.-, appearance of usual surface; B, appearance of surface of the raised margin of yellow spot; C, surface of central spot.

Distinctive Functions of the Layers.-As the distinctive functions of the several sub-layers of the middle. layer (granular and nuclear) are unknown, we will treat of only the three layers—inner, middle, and outer. The outer layer of rods and cones (bacillary) is undoubtedly the true receptive layer, which corresponds to the iodized film of the sensitized plate of the camera. These rods and cones receive and respond to the vibrations of light; they co-vibrate with the undulations of the ether. The inner or fibrous layer conducts the received impression to the optic nerve; for each rod and cone is connected by a slender thread, continuous with nucleated cells of the granular layer and a fiber of the fibrous layer. The fibrous layer may, in fact, be regarded as a layer of conducting threads coming from the rods and cones, which threads are then gathered into a cord or cable, the optic nerve, which in its turn finally conducts the impression to the brain. The function of the middle layer is more obscure; but nucleated nerve-cells, and especially multipolar cells, are always generators or

originators of nerve-force. They evidently have an important function. They probably act as little nervecenters; and many unconscious, involuntary, or reflex acts of vision are probably performed by their means, without referring the sensation to the brain.

The manner in which the whole apparatus operates is briefly as follows: The light penetrates through the retina until it reaches the outer layer of rods and cones. These are specially organized to respond to or co-vibrate with the undulations of light. These vibrations are carried through the connecting threads to the fibrous layer, then through the fibers of this layer to the optic nerve, then along the fibers of the optic nerve to the gray matter of the brain, where they finally determine changes which emerge into consciousness as the sensation of light.

That we have correctly interpreted the function of the layer of rods and cones is rendered probable not only by its very remarkable and complex structure, adapting it to responsive vibrations, but also by the peculiar properties of two spots on the retina on which all the layers do not co-exist. Just where the optic nerve enters, as shown in Fig. 21, page 55, the bacillary layer is necessarily wanting, and it is the only spot in which this is the case. Now, this spot is blind (see page 78). Again, just in the axis of the globe, or what might be called the south pole of the eye, is the central spot or central pit. In this spot is wanting the fibrous layer and the whole of the middle layer, except the multipolar cells. The bacillary layer is here, therefore, directly exposed to the action of light. Now, this is the most sensitive spot of the retina.

Perception of Color.-Color, like musical pitch, consists of an infinite number of kinds and shades; but

these may be reduced to a few primary kinds, by the mixture of which the intermediate shades may be supposed to be made. Newton made seven primary colors in the solar spectrum; but though these, and indeed many more, may be considered distinct from the physical point of view, since they are the result of different rates of ethereal vibration, yet they can not be all considered as primarily distinct sensations. Brewster reduced all color-sensations to three primary, viz., red, yellow, and blue. Young made them red, green, and violet. This latter view is adopted by Helmholtz and most modern writers.

Recently, however, Hering* has reinvestigated the whole subject with great acuteness, from the purely physiological instead of physical point of view, and arrives at different results. Hering includes white and black among his primary color-sensations, making six in all. But, leaving out these as belonging rather to the category of shades or nuances, according to Hering there are four and only four primary color-sensations essentially distinct from each other, viz., red, yellow, green, and blue. Aside from all physical considerations, undoubtedly this is true. These four colors are essentially distinct and irresolvable into any mixture of others. Again, according to Hering, these four are reducible to two complementary pairs, viz., red and green on the one hand, and yellow and blue on the other. This is also undoubtedly true. Finally, according to Hering, complementary colors are the result of opposite affections of the retina, so that there are only two essentially distinct color-affections of the retina, which, with their opposites, produce the two pairs of complementary colors: the one with its opposite produces red and green;

* Hering, "Zur Lehre von Licht-Sinne," Wien, 1878.

the other with its opposite, yellow and blue. This, though more doubtful, seems a probable cause of complementariness.

Theory of Color-Perception.-Color-perception is undoubtedly a simple perception, and irresolvable into any other. It must, therefore, have its basis in retinal structure. Since light is perceived by co-vibration of retinal elements, and since the different colors have different rates of vibration, there must be a corresponding structure of the retinal elements, by means of which they co-vibrate with each of these colors. In the ear different rates of aërial vibration (musical pitch) are perceived by means of rods of different lengths (rods of Corti), which co-vibrate, each with its own pitch. It seems probable, therefore, that different rods or cones covibrate with different rates of ethereal undulations, i. e., with different colors. This is the commonly received view, brought forward first by Young. It is supposed that there are three kinds of rods or cones, which severally co-vibrate with the three primary colors of Young. One kind responds to the slower vibrations of red, another kind to those of green, and still another to the more rapid vibrations of violet. When two kinds vibrate, intermediate colors are perceived. When all vibrate together, then white light is perceived. Or, to express it differently, intermediate colors produce vibration of two kinds, white light of all kinds, of rods. Or, if we adopt the theory of Hering in regard to the primary colors, one kind of rod or cone responds to red and green, another kind to yellow and blue.

Very recently Stanly Hall has proposed a theory which seems even more probable.* He believes that color is perceived by the cones alone; further, that

*“American Academy of Science and Art," vol. xiii, p. 402 (1878).