Of the four per cent. of males who suffer from congenital colour deficiency of vision, a large number are not totally lacking in any one or more colour sensations. Those in which one sensation, on the Young theory, is entirely missing are called “completely red-, green-, or violet-blind,” whilst those in which the sensation is but partially deadened are called “partially red-, green-, or violet-blind.” When two sensations are entirely absent, and such cases are very rare indeed, they are generally said to have monochromatic vision; that is, every colour to them is the same, as is also white, the only distinction between any of them being the superior brightness of some over others. The best illustration of this form of colour vision is perhaps to say that the retinæ of such people have the same characteristics in regard to sensitiveness as has a photographic plate, the resulting prints in black and white representing what it sees in nature. When we have to adopt the terms used by the followers of Hering’s theory--the theory which obtains most followers amongst the physiologists, since it endeavours to explain colour vision in a physiological way, though it fails to meet all the requirements of the physicist--we should restrict our terms to red-green and yellow-blue blindness, still perhaps retaining the term monochromatic vision for the rare cases specified above. As we must employ some terms to express our meaning, we shall in these lectures adopt those of the Young theory.