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.
The account of Dr. Dalton's own peculiarity of vision by himself, offers considerable interest. He says, speaking of flowers: "With respect to colours that were white, yellow, or green, I readily assented to the appropriate term; blue, purple, pink, and crimson appeared rather less distinguishable, being, according to my idea, all referable to blue. I have often seriously asked a person whether a flower was blue or pink, but was generally considered to be in jest." He goes on further to say, as the result of his experience: "1st. In the solar spectrum three colours appear, yellow, blue, and purple. The two former make a contrast; the two latter seem to differ more in degree than in kind. 2nd. Pink appears by daylight to be sky-blue a little faded; by candlelight it assumes an orange or yellowish appearance, which forms a strong contrast to blue. 3rd. Crimson appears muddy blue by day, and crimson woollen yarn is much the same as dark blue. 4th. Red and scarlet have a more vivid and flaming appearance by candlelight than by daylight" (owing probably to the quantity of yellow light thrown upon them).
As anecdotes concerning this curious defect of colour vision, we may quote also the following: "All crimsons appeared to me (Dr. Dalton) to be chiefly of dark blue, but many of them have a strong tinge of dark brown. I have seen specimens of crimson claret and mud which were very nearly alike. Crimson has a grave appearance, being the reverse of every showy or splendid colour." Again: "The colour of a florid complexion
