we may begin with visual and photographic observations with the telescope. Such remarkable photographs as that of the Andromeda nebula seem to bring us into the very presence of a greater system, more nearly comparable in size with the Milky Way than with the solar system, in the actual process of formation. But on account of the long periods of time, which must elapse before changes in this distant mass may become sufficiently great to be appreciable, and for many other reasons, we could not hope to base a complete scheme of stellar evolution on such photographs alone. Our observational methods must also include the means of solving physical, chemical and gravitational problems as they present themselves, not close at hand in the laboratory, but at inconceivably distant regions of space. For this reason it would have been impossible prior to the invention of the spectroscope to arrange the stars according to any clearly defined system of development. The principal advances which have been made in the study of stellar evolution are therefore confined to the period which has elapsed since the middle of the nineteenth century.
Thus the investigation of stellar evolution has been contemporaneous with the investigation of organic evolution. Indeed, the epoch-making discovery of the chemical composition of the sun by Kirchhoff and Bunsen was made in the year of the publication of the 'Origin of Species.' Before this discovery the meaning of spectral lines had been as obscure as the meaning of Egyptian hieroglyphs prior to the discovery of the Rosetta stone. After it the chemical analysis of a star became hardly less difficult than the analysis of an unknown substance in the laboratory. Furthermore, it soon became apparent that the light of a star, as decomposed by a prism, was competent to define the star's position in a general scheme of development, in which every advance, from the unformed nebulous cloud on through the highest degree of stellar brilliancy to such a final stage as is typified by the moon, can be defined with but little danger of error. Before we proceed to consider some of the evidences of stellar evolution, let us examine some of the instruments and methods without which the discoveries to be subsequently described would have been impossible.
I shall confine my remarks on modern astrophysical instruments to those at present employed at the Yerkes Observatory, partly because nearly all the celestial photographs reproduced in the figures were taken with these instruments and partly because of the convenience of illustrating them. But before describing the great telescope which forms the principal apparatus of the observatory, I wish to point out that many of the most important results of astronomy, results which could not be obtained with a powerful telescope for the very reason of its great power—have been derived from the use of an ordinary camera, with just such a lens