Now taking a red-blind person and examining him with the spectrum, we find that he sees no light at all at the extreme limit of our red, and only when he comes to the part where the red lithium line marks a certain red does a glimmer commence; he then sees what he may call dark-green, or he may call dark-yellow. When questioned about what to us are greens he also calls them green or yellow, some being bright, others saturated hues, and others again paler. When he gets to the bluish-green he calls it grey, and will say it is indistinguishable from, and in fact will match with, a white degraded in tone. From this point he will say he sees blue, near F pale-blue, and in the violet dark-blue. Too much importance must not be attached to the nomenclature adopted by the colour blind. They have to take the names of the colours from the normal eyed. Yellow objects are generally brighter than red, and having annexed the idea that what to them is bright red is called yellow, they give it that distinguishing name. His limit of vision at the violet end will be the same as the majority of mankind, but it will be considerably shortened at the red end. The point in the spectrum which he calls grey is an important point, and corresponds to the place where the violet and green curves cut in Fig. 16. This point can be very accurately determined by placing alongside the colour patch A (Fig. 6) the white patch, which is reduced in brightness as required by rotating sectors. As the slit is moved along the spectrum it will eventually reach a point where he will say both patches of light are exactly similar in hue. To the normal eye one will be white and the other the kind of green indicated above (see frontispiece).