Next let us take the case of an intelligent person affected with colour blindness, but who is not yet aware of the fact. He has been taught from childhood that certain shades, some darker and some brighter, but all of neutral tint, and not really presenting to him colour at all, are to be called by various names—scarlet, crimson, pale red, dark red, bright red, dark green, dark purple, brown, and others. With all these he can only associate an idea of gray; nor can he possibly know that any one else sees more than he does. Having been taught the names they are called by, he remembers the names, with more or less accuracy, and thus passes muster. There is a real difference of tint, because each of these colours consists of more or less blue, yellow, and white, mixed with the red; and our friend is enabled to recognize and name them, more or less correctly, according to his acuteness of perception and accuracy of memory. If we desire to experiment on such a person, we must ask no names whatever, but simply place before him a number of similar objects differently coloured. Taking, for example, skeins of coloured wools, let us select a complete series of shades of tint, from red, through yellow, green, and blue, to violet, and request him to arrange them as well as he is able, placing the darkest shades first, and putting those tints together that are most like each other. It is curious then to watch the progress of the arrangement. In a case lately tried by the writer of this article, the colour-blind person first threw aside at once a particular shade of pale green as undoubted white, and then several dark blues, dark reds, dark greens, and browns, were put together as black. The yellows and pure blues were placed correctly, as far as name was concerned, by arranging several shades in order of brightness—but the order was very different from that which another person would have selected. The greens were grouped, some with yellows, and some with blues. The colours in this experiment were all negative and impure, but we may also obtain something like the same result with positive colour, transmitted by the aid of polarized light through plates of mica. In a case of this kind described by Sir J. Herschel, the only colours seen were blue and yellow, while pale pinks and greens were regarded as cloudy white, fine pink as very pale blue, and crimson as blue; white red, ruddy pink, and brick red were all yellows, and fine pink blue, with much yellow. Dark shades of red, blue, or brown, were considered as merely dark, no colour being recognized.