After some remarks on the peculiar difficulties which attend investigations into the functions of the eye, the author observed, that by far the most remarkable variety of colour-blindness, in a scientific point of view, is that which shows itself in the identification of red with black. This appeared to have been overlooked by previous observers, or at least only cursorily described. The probable causes of this neglect were noticed; and the author then proceeded to detail the experience of some twelve parties by whom various objects of a red, crimson, or scarlet colour were mistaken for black, and appeared, from the testimony of those who committed the mistakes in question, to have made neither a colorific nor a luminous impression on the retina. It was further shown, that though the fact had not attracted attention, the published cases of colour-blindness supplied examples of the same blindness to red; and that Dalton, although he had apparently ascertained his own freedom from the blindness in question, had incidentally supplied proof that the red alike of the solar spectrum and of coloured objects frequently appeared to him as dark or nearly black.