The Pole Star (Polaris) is a double star, one of the components being of a little below the second magnitude, and the other a little below the ninth. They are so close together that they appear as one star to the naked eye, but they may be seen separate with a moderately sized telescope. The brighter star is yellowish, and the faint one white. This brighter star is found _by means of the spectroscope_ to be actually composed of three stars so very close together that they cannot be seen separately even with a telescope. It is thus a triple star, and the three bodies of which it is composed are in circulation about each other. Two of them are darker than the third. The method of detecting binary stars by means of the spectroscope is an application of Doppler's principle. It will, no doubt, be remembered that, according to the principle in question, we are enabled, from certain shiftings of the lines in the spectrum of a luminous body, to ascertain whether that body is approaching us or receding from us. Now there are certain stars which always appear single even in the largest telescopes, but when the spectroscope is directed to them a spectrum _with two sets of lines_ is seen. Such stars must, therefore, be double. Further, if the shiftings of the lines, in a spectrum like this, tell us that the component stars are making small movements to and from us which go on continuously, we are therefore justified in concluding that these are the orbital revolutions of a binary system greatly compressed by distance. Such connected pairs of stars, since they cannot be seen separately by means of any telescope, no matter how large, are known as "spectroscopic binaries." In observations of spectroscopic binaries we do not always get a double spectrum. Indeed, if one of the components be below a certain magnitude, its spectrum will not appear at all; and so we are left in the strange uncertainty as to whether this component is merely faint or actually dark. It is, however, from the shiftings of the lines in the spectrum of the other component that we see that an orbital movement is going on, and are thus enabled to conclude that two bodies are here connected into a system, although one of these bodies resolutely refuses directly to reveal itself even to the all-conquering spectroscope.