There is another class of binary stars, in which one of the companions is invisible, its presence being indicated by the effects of its gravitational pull upon the other. Algol may be regarded as an example of this kind of stellar association. But there are stars of this class, where the companion causes no eclipses, either because it is not dark, or because it never passes over the other, as seen from the earth, but where its existence is proved, in a very interesting way, by the spectroscope. In these stars, called spectroscopic binaries, two bright components are so close together that no telescope is able to make them separately visible, but when their plane of revolution lies nearly in our line of sight the lines in their combined spectrum are seen periodically split asunder. To understand this, we must recall the principles underlying spectroscopic analysis and add something to what was said before on that subject.