Lying here is a layer of what are known as rods and cones, which have a connection, either actual or functional, with the optic fibres which largely compose the inner wall of the retina, and are connected with the optic nerve. In the yellow spot the cones are much more numerous than the rods, but in the peripheral part the reverse is the case. In the fovea the rods appear to be altogether absent. The total number of cones in the eye has been calculated to be about 3,000,000, of which about 7,000 are in the small fovea. The number of cones will give an idea of their dimensions. This detail has been entered into as it has been supposed that these rods and cones are all-important in translating light-waves into visual impulses. The inner wall of the retina of most human eyes, as has been mentioned, is stained with a black pigment, fuscin, though in albinos it is absent. What its particular use may be is still unknown, for its change by light is so slow that it can scarcely be the cause of vision. In the outer parts of the rods is, however, diffused a substance highly sensitive to light, called the “visual purple,” from its colour, and a theory founded on chemical action, produced by a change in this substance, has been promulgated. Fascinating, however, as such a theory must be, it lacks confirmation. The fact that the cones do not contain it, and that in the fovea are cones alone, renders it difficult to reconcile the theory with the fact that this part of the retina possesses, we are told, the greatest acuteness of sensation as regards light and colour. The eyes of most vertebrate animals, it may be remarked, have this visual purple, but in those of the bat, owl, hen, and some others the colouring matter seems to be absent. Visual purple is an interesting substance, however, and as it is found in the eye it probably exercises some useful function, though what that function may be is at present unknown. That images of objects can be formed on the retina, owing to the bleaching of this substance, has been proved by experiment. The purple is first changed to a yellow colour, and then passes into white. These “optograms,” as they are called, can be fixed in an excised eye if the retina be detached, and then be treated with a weak solution of alum. [Illustration: FIG. 3.]