sion has passed beyond the retina, there is no special or important difference m the views as to the final conversion into a sensation The objections to these two hypotheses we have already stated. The acceptance of such an hypothesis as we propose, however, does not involve the necessity of inventing new laws, or of creating new issues, but only applies known laws and analogous reactions of other substances to the elucidation of the phenomena observed. We know that there are membranes which respond with promptness to any number of simple aerial vibrations at the same time, and recent discoveries have shown that there are substances which, when in proper condition thus respond to wave-lengths of light. Silenium, when in a crystalline condition, alters its molecular condition (as manifested by its varying resistance to the passage of the electric current), not only when acted on by light of varying intensity, but also by the different wave-lengths If, then, we suppose the retina to be a substance of this nature but responding more promptly, and in a more delicate manner, than any other known substance to the wave-lengths of light, we have a basis for a theory of vision which is extremely simple in its nature, and founded on known physical laws.
We will not here enter upon a detailed application of this theory to the elucidation of all the phenomena of colored vision, but will simply mention a few points in connection with color-blindness. One general principle may be laid down which will cover all cases of retinal color-blindness as distinguished from cerebral or central, and that is, that in these cases the molecular structure of the retina is so altered as to allow it to respond feebly or not at all to light rays of certain wave-lengths. We know, for example, that silenium must be in a crystalline state—that is, its molecular structure must be in a certain definite condition—before it can respond in such a delicate manner to variation m the intensity of the light-waves; and we know that there are certain wave-lengths of the ether—the ultra-red and the ultra-violet—which call forth no reaction on the part of the retinal substance. It would, therefore, be a highly justifiable supposition that a slight alteration m the molecular structure of the retina would render it incapable of being affected by certain wave-lengths to which it, when in a normal condition, readily responds. This incapability may be partial or complete as regards any particular wave-lengths. In some instances of color-blindness, for example, the spectrum is shortened at the red end even under the most intense illumination, while in others there is a shortening only when the illumination is feeble—becoming of normal length when the intensity of the illumination is increased—showing, in the latter case, that the reaction to the red rays is still present when they are of sufficient intensity.
When we come to cerebral color-blindness, which is, according to my view, the most common, the explanation is still simple. In this instance we have only to suppose the cerebral center of vision incapa-