red lights
as they emerge from the glass prism, we allow them to fall on a screen,
we shall find that they pass, by continuous gradations, from red at the
one end of the screen, through orange, yellow, green, blue, and indigo,
to violet at the other end. _In other words, what we call white light is
composed of rays of these several colours. They go to make up the effect
which we call white._ And now just as water can be split up into its two
elements, oxygen and hydrogen, so sunlight can be broken up into its
primary colours, which are those we have just mentioned.

This range of colours, produced by the spectroscope, we call the solar
spectrum, and these are, from the spectroscopic point of view, primary
colours. Each shade of colour has its definite position in the spectrum.
That is to say, the light of each shade of colour (corresponding to its
wave-length) is reflected through a certain fixed angle on passing
through the glass prism. Every possible kind of light has its definite
position, and is denoted by a number which gives the wave-length of the
vibrations constituting that particular kind of light.

Now, other kinds of light besides sunlight can be analysed. Light
from any substance which has been made incandescent may be observed with
the spectroscope in the same way, and each element can be thus
separated. It is found that each substance (in the same conditions of
pressure, etc.) gives a constant spectrum of its own. _Each metal
displays its own distinctive colour. It is obvious, therefore, that the
spectrum provides the means for identifying a particular substance._ It
was by this method that we discovered in the sun the presence of such
well-known elements as sodium, iron, copper, zinc, and magnesium.

[Illustration: _Yerkes Observatory._

FIG. 9.--THE GREAT SUN-SPOT OF JULY 17, 1905]

[Illustration: _From photographs taken at the Yerkes Observatory._

FIG. 10.--SOLAR PROMINENCES

These are about 60,000 miles in height. The two photographs show the
vast changes occurring in ten minutes. October 10, 1910.]

[Illustration: _Photo: Mount Wilson Observatory._

FIG. 11.--MARS, October 5, 1909

Showing the dark markings and the Polar Cap.]

[Illustration: FIG. 12.--JUPITER

Showing the belts which are probably cloud formations.]

[Illustration: _Photo: Professor E. E. Barnard, Yerkes Observatory._

FIG. 13.--SATURN, November 19, 1911

Showing the rings, mighty swarms of meteorites.]

Every chem