s, as given in Fig. 104. Nos. 1 and
2 have one flat and one spherical surface. Nos. 3, 4, 5, 6 have two
spherical surfaces. When a lens is thicker at the middle than at the
sides it is called a _convex_ lens; when thinner, a _concave_ lens. The
names of the various shapes are as follows:--No. 1, plano-convex; No. 2,
plano-concave; No. 3, double convex; No. 4, double concave; No. 5,
meniscus; No. 6, concavo-convex. The thick-centre lenses, as we may term
them (Nos. 1, 3, 5), _concentrate_ a pencil of rays passing through
them; while the thin-centre lenses (Nos. 2, 4, 6) _scatter_ the rays
(see Fig. 105).

[Illustration: FIG. 104.--Six forms of lenses.]

THE CAMERA.

[Illustration: FIG. 105.]

[Illustration: FIG. 106.]

We said above that light is propagated in straight lines. To prove this
is easy. Get a piece of cardboard and prick a hole in it. Set this up
some distance away from a candle flame, and hold behind it a piece of
tissue paper. You will at once perceive a faint, upside-down image of
the flame on the tissue. Why is this? Turn for a moment to Fig. 106,
which shows a "pinhole" camera in section. At the rear is a ground-glass
screen, B, to catch the image. Suppose that A is the lowest point of the
flame. A pencil of rays diverging from it strikes the front of the
camera, which stops them all except the one which passes through the
hole and makes a tiny luminous spot on B, _above_ the centre of the
screen, though A is below the axis of the camera. Similarly the tip of
the flame (above the axis) would be represented by a dot on the screen
below its centre. And so on for all the millions of points of the flame.
If we were to enlarge the hole we should get a brighter image, but it
would have less sharp outlines, because a number of rays from every
point of the candle would reach the screen and be jumbled up with the
rays of neighbouring pencils. Now, though a good, sharp photograph may
be taken through a pinhole, the time required is so long that
photography of this sort has little practical value. What we want is a
large hole for the light to enter the camera by, and yet to secure a
distinct image. If we place a lens in the hole we can fulfil our wish.
Fig. 107 shows a lens in position, gathering up a number of rays from a
point, A, and focussing them on a point, B. If the lens has 1,000 times
the area of the pinhole, it will pass 1,000 times as many rays, and the
image of A will be impressed on a sensitize