The last great path-breaking discovery which has helped us in our understanding of the stars was made by Fraunhofer and other physicists, who showed us that substances when in a heated, gaseous, or vaporous state produced, in a way which it is not easy to explain in a work such as this, certain dark lines in the spectrum, or streak of divided light which we may make by means of a glass prism, or, as in the rainbow, by drops of water. Carefully studying these very numerous lines, those naturalists found that they could with singular accuracy determine what substances there were in the flame which gave the light. So accurate is this determination that it has been made to serve in certain arts where there is no better means of ascertaining the conditions of a flaming substance except by the lines which its light exhibits under this kind of analysis. Thus, in the manufacture of iron by what is called the Bessemer process, it has been found very convenient to judge as to the state of the molten metal by such an analysis of the flame which comes forth from it. [Illustration: _Seal Rocks near San Francisco, California, showing slight effect of waves where there is no beach._] No sooner was the spectroscope invented than astronomers hastened by its aid to explore the chemical constitution of the sun. These studies have made it plain that the light of our solar centre comes forth from an atmosphere composed of highly heated substances, all of which are known among the materials forming the earth. Although for various reasons we have not been able to recognise in the sun all the elements which are found in our sphere, it is certain that in general the two bodies are alike in composition. An extension of the same method of inquiry to the fixed stars was gradually though with difficulty attained, and we now know that many of the elements common to the sun and earth exist in those distant spheres. Still further, this method of inquiry has shown us, in a way which it is not worth while here to describe, that among these remoter suns there are many aggregations of matter which are not consolidated as are the spheres of our own solar system, but remain in the gaseous state, receiving the name of nebulae.