The term periscopic is applied to lenses intended for spectacles, which are uncorrected for colour or non-achromatic. For use in a camera they are, of course, much cheaper than the proper achromatic combination, but will never give a sharp image. The reason for this is as follows. When a ray of light is refracted or bent (as happens when it passes through a prism; see Fig. 1), this ray is split up into its component parts, that is, into rays of different colour. (It is, of course, well known that the impression produced by a ray of white light is the combination of the sensations produced by each of the different coloured rays.) The violet rays are bent most, and the red rays are bent least: that is to say, the violet rays cross each other or come to a focus nearest the lens (see Fig. 2), and the red rays cross or come to a focus at a point farthest from the lens, blue and yellow being focussed at different points between the violet and the red. Now, if two prisms are put base to base a diagrammatic or crude kind of lens is formed; and if the courses of two rays r are traced (see Fig. 2), the explanation given above will be intelligible. Thus the violet rays which crossed at V will have spread out again sufficiently at the point R, at which the red rays are focussed, to form a halo or confused disc around each point of violet light, and the red rays form a similar halo at V. The principal rays used in ordinary vision are nearer the red end of the spectrum or colour scale, whilst the rays that are most active, chemically, are at the violet end of the scale, hence the terms visual and chemical foci. In using a periscopic lens, therefore, it is necessary after focussing to rack in the screen usually about one-sixteenth of the focus. Spectacle lenses may be purchased for threepence each, and if used on suitable subjects will yield very satisfactory results. The defects of these lenses are greatly reduced by using a small stop. An achromatic lens consists of two lenses, one of flint (lead) glass and one of crown (soda) glass, cemented together. It is found that although both lenses may disperse the colours equally, yet they have different refractive indices, or bend the rays to a different extent.

In Figs. 1 and 3, although the angles of incidence A and the angles of dispersion A' are equal each to each, the angle B is greater than B'. If, therefore, the prisms be placed in opposite positions, the dispersion of one prism will be neutralised by the dispersion of the other, but the ray will proceed as shown in Fig. 4. In all the diagrams, R indicates red rays, and V violet rays.

Rectilinear and Periscopic Lenses.