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far hotter even than the sun. This is true of their surfaces and must be still more true of their vast interiors.

Stars are not the only bodies which fill these distant regions of space. Scattered over the sky are immense masses of exceedingly rare matter which, from their cloud-like appearance, are called nebulæ. In size these bodies far exceed the sun or stars. A nebula only as large as our solar system would probably be invisible in the most powerful telescope, and could never be impressed even on the most delicate photograph of the sky unless above the ordinary brightness. Those that we know have probably hundreds or thousands of times the extent of our whole solar system. We may therefore classify those bodies of the universe which shine by their own light as stars and ebulæ.

When we read of astronomical discoveries, we commonly think of them as being made by looking through a telescope. But this is no longer the case. The greatest astronomical development of recent times consists in proving the existence of dark bodies of the nature of planets, revolving around many stars. These objects are absolutely invisible in any telescope which it would be possible to construct. Such an instrument could tell us nothing about the constitution of a star. The great engine of progress has been the spectroscope, which is described in a previous chapter. From what has there