When one portion of the retina is excited by the luminous rays, the vibration is extended to the neighbouring portions, and more strongly in proportion as the light is white; the result of this is, that of two objects of equal dimensions but of different colour, the lighter one in colour appears the larger in size. 1 a black circle is traced on a sheet of white paper, and a white circle of the same size on a sheet of black paper, and both placed at an equal distance from the eye, the white one appears larger than the black. In the same way, if we make a disk half white and half black, the white half appears the larger. In both cases the white encroaches upon the black because the impression made by it upon the retina is more vivid, and the longer the experiment is continued the greate appears the difference in diameter. The name of irradiation, has been given to the group of phenomena of this nature. It is the same cause which produces a ring of complementary colour around an image impressed on the retina by a coloured object. If a square of red be placed upon a white ground and the eyes are fixed upon it for a time, a border of pale green forms itself round the red; and in the same way a yellow square on a white ground produces a blueish crown round the yellow image: these are called accidental fringes of light.
M. Chevreul has discovered some remarkable laws which govern the contrast of colours, and the mutual influence which two colours placed in juxtaposition have upon each other. The investigations of the eminent professor are not more important for the arts than for science, for the phenomena of irradiation are produced constantly in vision, and artists should not forget them for a moment in painting or in architecture. It is unnecessary to remark that the harmonious or discordant effect produced by the association of colours in these two arts is of the utmost importance, and although in general the spectator troubles himself very little about the law of contrasts, yet he is notwithstanding very sensible of the impressions which result from its observance.
The effects of a disturbance in vision described for the first time by an English chemist who was attacked by it, are commonly designated by this term. It consists of a difficulty, more or less great, of distinguishing colours, some of which are entirely confounded although very different, as rose and gray, red and green, etc. Very marked cases of Daltonism are rarely met with, but in a slight degree the affection is not uncommon.