The imperfections are chiefly two, viz. those known as chromatic and spherical aberration respectively. Chromatic aberration simply means that all the colours composing the original beam of, say, white light are not equally refracted or converged, and therefore do not meet again at the same spot (the well-known prism or lustre effect), and reveals itself by coloured fringes round the edges of the various details in the picture.
By spherical aberration we mean that the light falling upon the centre of a lens is not brought to a focus at exactly the same spot as the marginal rays, and a general want of definition is the result, usually accompanied also by a want of 'flatness' in the image, that is to say the edges of the picture do not focus at the same time as the centre.
Chromatic aberration is easily cured by using an achromatic or compound lens made by cementing together two lenses of crown and flint glass respectively, as in Fig. 40.
It will be seen that the flint glass component by itself is a concave lens and therefore neutralises in part, or in whole, the convex crown lens. Flint glass has both greater dispersive power and also greater refractive power than crown glass, but fortunately not to the same degree; hence a compound lens made in this way and with curves carefully worked out may have its chromatic effect entirely neutralised while retaining very considerable refractive or 'focussing' power, and simple achromatic objectives of this type are quite inexpensive.
In lanterns intended for Science demonstration, as distinct from the mere projection of slides, lenses of this pattern are very frequently used, as they will project the latter when required reasonably well, and for the demonstration of
experiments or of apparatus on the screen have advantages that need not be discussed here.
For very long focus lenses also they are sometimes employed, as the trouble from spherical aberration is much less apparent with lenses of long focus than with short, and the difference in expense is much more in the former case than in the latter. For short focus lenses, however, as used in moderate-sized halls, they are not good enough, and the type of lens almost universally employed is that known as the 'Petzval' combination (Fig. 41).
