The branch of physics which at the present day has assumed such mighty and far-reaching proportions in astronomical work is that dealing with spectrum analysis, which, although suggested as early as the time of Kepler, did not receive any impetus as regards its application to celestial bodies until the beginning of the present century at the hands of Wollaston and Fraunhofer. Then, however, it still lacked the chemical touch supplied afterwards by Kirchhoff and Bunsen. They showed us that the spectrum observed when the light from any heated body is passed through a prism is an index to the chemical composition of the light source; the constitution of a vapor when in a condition to absorb light can be determined by an extension of the same principle, first demonstrated by Stokes, Angström, and Balfour Stewart, when the century was about half completed. The first celestial body towards which the spectroscope was turned was our central luminary, the sun. Wollaston first discovered that its spectrum was crossed by a few dark lines; we learned next from Fraunhofer, who in 1814 worked with instruments of greater power, that the solar spectrum was crossed not only by a few dark lines, but by some hundreds. Not content with examining the light of the sun, Fraunhofer turned his instrument towards the stars, the light of which he also examined, so that he may be justly called the inventor of stellar spectrum analysis. It is not to the credit of modern science that from this time forward spectrum analysis did not become a recognized branch of scientific inquiry, but, as a matter of fact, Fraunhofer’s observations were buried in oblivion for nearly half a century. The importance of them was not recognized till the origin of the dark lines, both in sun and stars, had been explained by Stokes and others, as before stated. The lines in the solar spectrum were mapped with great diligence by Kirchhoff in 1861 and 1862, and later by Angström and Thalen, and this was done side by side with chemical work in the laboratory. The chemistry of the sun was thus to a great extent revealed; it was no longer a habitable globe, but one with its visible boundary at a fierce heat, surrounded by an atmosphere of metallic vapors, chief among them iron, also in a state of incandescence. To these metallic vapors Angström added hydrogen shortly afterwards. Here, then, was established a firm link between the heavens and the earth; the first step to the problem of the chemistry of space had been taken.