In the course of those researches on Colour-Blindness, which, at intervals, I have recently been engaged in prosecuting, I have encountered some phenomena connected with normal vision, which I am desirous to submit to the consideration of the Society. Those phenomena I have already in part detailed, in the account of the researches referred to, and I shall not, accordingly, repeat the description of them here, to a greater extent than is essential to rendering intelligible the question which I wish to submit for discussion.I venture to assume, that without adducing a lengthened series of authorities, I may take for granted, that, on the received theory of vision, the eye of man, as well as that of most of the lower animals, is regarded as essentially realizing, during the performance of its function of sight, the condition of a darkened chamber, or camera obscura. In more precise words: the theory in question teaches, that those rays of light, which reach the eye from the objects which they render visible, and entering at its front traverse all its transparent humours and membranes, last of all pierce the retina, and after making that impression upon it which is supposed to be the most important physical element of vision, are stopped, or absorbed by the dark pigment lining the choroid coat, and suffer extinction as visible rays. The dark surface of the choroid is thus held to abolish all the light which reaches it, so that none of the luminous rays return through the retina, or retrace their course across the chamber of the eye.