lysing portion consists of a
single prism. Unless the prism is very large, however, only a small
degree of dispersion is obtained. It is obviously desirable, for
accurate analytical work, that the dispersion--that is, the separation
of the different parts of the spectrum--should be as great as possible.
The dispersion can be increased by using a large number of prisms, the
light emerging from the first prism, entering the second, and so on. In
this way each prism produces its own dispersive effect and, when a
number of prisms are employed, the final dispersion is considerable. A
considerable amount of light is absorbed in this way, however, so that
unless our primary source of light is very strong, the final spectrum
will be very feeble and hard to decipher.

Another way of obtaining considerable dispersion is by using a
_diffraction grating_ instead of a prism. This consists essentially of a
piece of glass on which lines are ruled by a diamond point. When the
lines are sufficiently close together they split up light falling on
them into its constituents and produce a spectrum. The modern
diffraction grating is a truly wonderful piece of work. It contains
several thousands of lines to the inch, and these lines have to be
spaced with the greatest accuracy. But in this instrument, again, there
is a considerable loss of light.

We have said that every substance has its own distinctive spectrum, and
it might be thought that, when a list of the spectra of different
substances has been prepared, spectrum analysis would become perfectly
straightforward. In practice, however, things are not quite so simple.
The spectrum emitted by a substance is influenced by a variety of
conditions. The pressure, the temperature, the state of motion of the
object we are observing, all make a difference, and one of the most
laborious tasks of the modern spectroscopist is to disentangle these
effects from one another. Simple as it is in its broad outlines,
spectroscopy is, in reality, one of the most intricate branches of
modern science.

BIBLIOGRAPHY

(The following list of books may be useful to readers wishing to pursue
further the study of Astronomy.)

BALL, _The Story of the Heavens_.
BALL, _The Story of the Sun_.
FORBES, _History of Astronomy_.
HINCKS, _Astronomy_.
KIPPAX, _Call of the Stars_.
LOWELL, _Mars and Its Canals_.
LOWELL, _Evolution of Worlds_.
MCKREADY, _A Beginner's Star-Book_.
NEWCOMB, _Popular Astronomy_