rainbow; a phenomenon caused, as everybody knows, by the sun's rays
being split up into their component parts.

This light travels in straight lines and, striking objects before us, is
reflected in all directions. Some of these rays passing through a point
situated behind the lenses of the eye, strike the retina. The
multiplication of these rays on the retina produces a picture of
whatever is before the eye, such as can be seen on the ground glass at
the back of a photographer's camera, or on the table of a camera
obscura, both of which instruments are constructed roughly on the same
principle as the human eye.

These rays of light when reflected from an object, and again when
passing through the atmosphere, undergo certain modifications. Should
the object be a red one, the yellow, green, and blue rays, all, in fact,
except the red rays, are absorbed by the object, while the red is
allowed to escape. These red rays striking the retina produce certain
effects which convey to our consciousness the sensation of red, and we
say "That is a red object." But there may be particles of moisture or
dust in the air that will modify the red rays so that by the time they
reach the eye they may be somewhat different. This modification is
naturally most effective when a large amount of atmosphere has to be
passed through, and in things very distant the colour of the natural
object is often entirely lost, to be replaced by atmospheric colours, as
we see in distant mountains when the air is not perfectly clear. But we
must not stray into the fascinating province of colour.

What chiefly concerns us here is the fact that the pictures on our
retinas are flat, of two dimensions, the same as the canvas on which we
paint. If you examine these visual pictures without any prejudice, as