llion movements
per second for violet rays to 400 billion for red rays.

If a beam of white light be passed through a prism it is resolved into
the seven visible colours of the spectrum--violet, indigo, blue, green,
yellow, orange, and red--in this order. The human eye is most sensitive
to the yellow-red rays, a photographic plate to the green-violet rays.

All bodies fall into one of two classes--(1) _Luminous_--that is, those
which are a _source_ of light, such as the sun, a candle flame, or a
red-hot coal; and (2) _non-luminous_, which become visible only by
virtue of light which they receive from other bodies and reflect to our
eyes.

THE PROPAGATION OF LIGHT.

Light naturally travels in a straight line. It is deflected only when it
passes from one transparent medium into another--for example, from air
to water--and the mediums are of different densities. We may regard the
surface of a visible object as made up of countless points, from each of
which a diverging pencil of rays is sent off through the ether.

LENSES.

If a beam of light encounters a transparent glass body with non-parallel
sides, the rays are deflected. The direction they take depends on the
shape of the body, but it may be laid down as a rule that they are bent
toward the thicker part of the glass. The common burning-glass is well
known to us. We hold it up facing the sun to concentrate all the heat
rays that fall upon it into one intensely brilliant spot, which speedily
ignites any inflammable substance on which it may fall (Fig. 103). We
may imagine that one ray passes from the centre of the sun through the
centre of the glass. This is undeflected; but all the others are bent
towards it, as they pass through the thinner parts of the lens.

[Illustration: FIG. 103.--Showing how a burning-glass concentrates the
heat rays which fall upon it.]

It should be noted here that _sunlight_, as we call it, is accompanied
by heat. A burning-glass is used to concentrate the _heat_ rays, not the
_light_ rays, which, though they are collected too, have no igniting
effect.

In photography we use a lens to concentrate light rays only. Such heat
rays as may pass through the lens with them are not wanted, and as they
have no practical effect are not taken any notice of. To be of real
value, a lens must be quite symmetrical--that is, the curve from the
centre to the circumference must be the same in all directions.

There are six forms of simple lense