those producing blue and yellow; so that on turning the eye away from the red object, and permitting a little white light to enter it, that part of the retina which received the red image will, in consequence of its diminished sensibility to that colour, and its exalted sensibility to blue and yellow, be able to perceive the two latter colours best, and by their mixture will give rise to a green image of the red object. The same thing will be observed with all the other colours; the secondary image or spectre always appearing of the complementary colour to the object from which the impression is obtained.
The duration and vividness of these impressions on the retina vary greatly in different individuals, and can be procured from almost any object. A person may, after looking steadily, and as often happens, unconsciously for a short time at printed or painted figures, on paper, porcelain, &c, see, on turning the head in some other direction, a life-sized or colossal spectre (the spectre appears larger the greater the distance of the surface against which it is seen), and there can be little doubt but that many of the reputed ghosts originate in this manner.
