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_. NEWCOMB, _The Stars: A Study of the Universe_. OLCOTT, _Field Book of the Stars_. PRICE, _Essence of Astronomy_. SERVISS, _Curiosities of the Skies_. WEBB, _Celestial Objects for Common Telescopes_. YOUNG, _Text-Book of General Astronomy_. II THE STORY OF EVOLUTION INTRODUCTORY THE BEGINNING OF THE EARTH--MAKING A HOME FOR LIFE--THE FIRST LIVING CREATURES Sec. 1 The Evolution-idea is a master-key that opens many doors. It is a luminous interpretation of the world, throwing the light of the past upon the present. Everything is seen to be an antiquity, with a history behind it--a _natural history_, which enables us to understand in some measure how it has come to be as it is. We cannot say more than "understand in some measure," for while the _fact_ of evolution is certain, we are only beginning to discern the _factors_ that have been at work.