The organ of vision is the eye; and _the eye is a little photographic camera_, with shutter, iris-diaphragm, self-adjusting lens, dark chamber, and self-renewing sensitive film. We are concerned only with the film, that is, with the =retina= or nervous network that lines the posterior half of the eyeball. It seems that _the retina is really made up of three interfused films_; for simplicity’s sake you may consider them as lying upon one another, just as three saucers might do if you piled them together. The oldest and largest film, the bottom saucer, gives us the sensations of _black and white_; the middlemost, somewhat smaller, gives us _blue and yellow_; and the topmost and smallest gives us a _purplish-red and a bluish-green_. The existence and size of the three films can be shown by experiment; for we are all totally colour-blind at the edge of the field of vision, and are blind to reds and greens for some distance further in toward the centre. There are also cases of inherited =colour-blindness=, in which the eye is blind either for all colours (total colour-blindness) or for red and green alone (partial colour-blindness); the latter form is fairly common, as is natural,—for the red-green film, being the last to come, might be expected to be the first to go. Partial colour-blindness was first brought to scientific notice by the English chemist John Dalton in 1798. Dalton was a Quaker, but made no objection to wearing the scarlet gown of a doctor of laws, because, as he said, “to me its colour is that of nature—the colour of those green leaves”; it is needless to remark that he did not see green either! The defect is practically important for pilots and signalmen, who have to distinguish red and green lights.