ke of simplicity we have taken the two extremes only. You observe that
the point R, in which the red rays meet, is much further from the lens
than is V, the meeting-point of the violet rays. A photographer very
seldom has to take a subject in which there are not objects of several
different colours, and it is obvious that if he used a simple lens like
that in Fig. 111 and got his red objects in good focus, the blue and
green portions of his picture would necessarily be more or less out of
focus.

[Illustration: FIG. 111.]

[Illustration: FIG. 112.]

This defect can fortunately be corrected by the method shown in Fig.
112. A _compound_ lens is needed, made up of a _crown_ glass convex
element, B, and a concave element, A, of _flint_ glass. For the sake of
illustration the two parts are shown separated; in practice they would
be cemented together, forming one optical body, thicker in the centre
than at the edges--a meniscus lens in fact, since A is not so concave as
B is convex. Now, it was discovered by a Mr. Hall many years ago that if
white light passed through two similar prisms, one of flint glass the
other of crown glass, the former had the greater effect in separating
the spectrum colours--that is, violet rays were bent aside more suddenly
compared with the red rays than happened with the crown-glass prism.
Look at Fig. 112. The red rays passing through the flint glass are but
little deflected, while the violet rays turn suddenly outwards. This is
just what is wanted, for it counteracts the unequal inward refraction
by B, and both sets of rays come to a focus in the same plane. Such a
lens is called _achromatic_, or colourless. If you hold a common
reading-glass some distance away from large print you will see that the
letters are edged with coloured bands, proving that the lens is not
achromatic. A properly corrected photographic lens would not show these
pretty edgings. Colour correction is necessary also for lenses used in
telescopes and microscopes.

SPHERICAL ABERRATION.

A lens which has been corrected for colour is still imperfect. If rays
pass through all parts of it, those which strike it near the edge will
be refracted more than those near the centre, and a blurred focus
results. This is termed _spherical aberration_. You will be able to
understand the reason from Figs. 113 and 114. Two rays, A, are parallel
to the axis and enter the lens near the centre (Fig. 113). These meet in
one plane. Two ot