her rays, B, strike the lens very obliquely near the
edge, and on that account are both turned sharply upwards, coming to a
focus in a plane nearer the lens than A. If this happened in a camera
the results would be very bad. Either A or B would be out of focus. The
trouble is minimized by placing in front of the lens a plate with a
central circular opening in it (denoted by the thick, dark line in Fig.
114). The rays B of Fig. 113 are stopped by this plate, which is
therefore called a _stop_. But other rays from the same point pass
through the hole. These, however, strike the lens much more squarely
above the centre, and are not unduly refracted, so that they are brought
to a focus in the same plane as rays A.

[Illustration: FIG. 113.]

[Illustration: FIG. 114.]

DISTORTION OF IMAGE.

[Illustration: FIG. 115.--Section of a rectilinear lens.]

The lens we have been considering is a single meniscus, such as is used
in landscape photography, mounted with the convex side turned towards
the inside of the camera, and having the stop in front of it. If you
possess a lens of this sort, try the following experiment with it. Draw
a large square on a sheet of white paper and focus it on the screen. The
sides instead of being straight bow outwards: this is called _barrel_
distortion. Now turn the lens mount round so that the lens is outwards
and the stop inwards. The sides of the square will appear to bow towards
the centre: this is _pin-cushion_ distortion. For a long time opticians
were unable to find a remedy. Then Mr. George S. Cundell suggested that
_two_ meniscus lenses should be used in combination, one on either side
of the stop, as in Fig 115. Each produces distortion, but it is
counteracted by the opposite distortion of the other, and a square is
represented as a square. Lenses of this kind are called _rectilinear_,
or straight-line producing.

We have now reviewed the three chief defects of a lens--chromatic
aberration, spherical aberration, and distortion--and have seen how they
may be remedied. So we will now pass on to the most perfect of cameras,

THE HUMAN EYE.

The eye (Fig. 116) is nearly spherical in form, and is surrounded
outside, except in front, by a hard, horny coat called the _sclerotica_
(S). In front is the _cornea_ (A), which bulges outwards, and acts as a
transparent window to admit light to the lens of the eye (C). Inside the
sclerotica, and next to it, comes the _choroid_ coat; and insid