The retina is that part of the eye by which the physical motions called light are changed into what are known physiologically as nerve impulses, by means of which the impression of light is excited in the brain. In reaching the retina the light is not altered from the light with which physicists experiment, but at the retina this physical motion is stopped. The optic nerves no more convey the light waves from the eye to the brain than the tactile nerves carry the objects that stimulate their endings. They only send a nerve impulse which the retina, on its exposure to the light, excites in the terminals of the optic nerve. Any form of stimulation, if applied to the optic nerve, will cause an impulse to pass to the brain, which there sets up the sensation of light. Thus, we are told by persons who have had their optic nerves cut that the section was accompanied by the sensation of a flash of light but not pain. Any violent injury of the eyeball causes a flash of light to be experienced. This fact has longsince been recognized in a practical manner, for a blow implicating the eyeball is vulgarly said to "make one see stars." Also, without violent injury, if we close the eyes and turn them to the one side and then press through the lid with the point of a pencil on the other side of the eyeball, we have a sensation of a point or ring of light from the retinal stimulation. Thus we say that the specific energy of the optic nerves excites a sensation of light, and the adequate stimulus of the nerve terminals of the organ of vision is light. The first question that arises is, What part of the retina does this important work of stimulating the optic nerve when light impinges on its terminals?