_Dispersion, or Chromatic Aberration._--The rays of light have so far been considered as simple. They are, however, in reality compound, consisting of a number of primary-coloured rays, of which seven kinds are easily distinguishable, viz. red, orange, yellow, green, blue, indigo, and violet. The coloured rays of the sun are, as is well known, often seen separated by the action of the triangular glass bars or prisms forming the lustres of a chandelier; the separation arising from the different refrangibility of the coloured rays, by which each is refracted to a different degree from that of the others. This is shown in Pl. XII. fig. 19, representing a ray of white light entering a triangular prism, at the surface of which the paths of the rays become different according to the degree of their refrangibility, whence they emerge separately, forming a _spectrum_ at _v r_; the most refrangible violet rays (_v_) being most refracted, the less refrangible red (_r_) least so, the intermediate rays being refracted to intermediate degrees according to their respective refrangibilities. This separation of the coloured rays is called dispersion; and as different substances or media disperse the coloured rays over a larger or smaller space, so as to produce spectra of different lengths, they are said to possess different dispersive powers. Thus the dispersive power of flint glass and balsam are about equal, while that of crown glass is considerably less. The extent to which dispersion is produced by the same medium also depends upon the angle of the prism, being greater as the angle is larger; increased obliquity of the incident light also increases the dispersion, so that the spectrum produced by a small prism may be equal to that produced by a larger one upon which the light is less obliquely incident. In consequence of the dispersion of light, rays passing through a convex lens do not meet at a single point or focus (Pl. XII. fig. 20), but form as many foci as there are coloured rays. When the spectrum is received upon a convex lens, the coloured rays are brought to a focus, and the light appears again white; for it is only when the primary-coloured rays are parallel, and seen close together, that they produce the impression of white or colourless light. The spectra produced by different dispersive media not only differ in length, but also in the breadth of the coloured spaces not being in the same ratio to each other; hence the spectra are said to be irrational, or the dispersion is said to be irrational. _Vision._--The visibility of an object depends upon the rays of light which emanate from each point of its surface presented to the eye being brought to a focus upon the ret´ina or expansion of the nerve of sight lining the inside of the back of the eye; so that, an image of each point being impressed upon the retina, the sum of the images forms the compound image of the entire object.