Nearly twenty years had elapsed after Bessel had predicted the disturber of Sirius before the telescopic discovery which confirmed it was made. The circumstances under which that discovery was made are not, indeed, so dramatic as those which attended the discovery of Neptune, but yet they have an interest of their own. In February, 1862, Alvan Clark & Sons, the celebrated telescope-makers, of Cambridge, Massachusetts, were completing a superb eighteen-inch object-glass for the Chicago Observatory. Turning the instrument on Sirius, for the purpose of trying it, the practiced eye of Alvan Graham Clark, the son, soon detected something unusual, and he exclaimed, “Why, father, the star has a companion!” The father looked, and there was a faint companion due east from the bright star, and distant about ten seconds of a degree. This was exactly the predicted direction of the companion of Sirius, and yet the observers knew nothing of the prediction. As the news of this discovery spread, many great telescopes were pointed on Sirius; and it was found that when observers knew exactly where to look for the object, many instruments would show it. The new companion star to Sirius lay in the true direction, and it was now watched with the keenest interest, to see whether it also was moving in the way it should move,--if it were really the body whose existence had been foretold. Four years of observation showed that this was the case, so that hardly any doubt could remain that the telescopic discovery had been made of the star which had caused the inequality in the motion of Sirius. It is certainly a very remarkable fact that, out of the thousands of stars with which the heavens are adorned, no single star has yet been found which certainly shows an appreciable disk in the telescope. We are aware that some skillful observers have thought that certain small stars do show disks; but we may lay this aside, and appeal only to the ordinary fact that our best telescopes turned on the brightest stars show merely glittering points of light, so hopelessly small as to elude our most delicate micrometers. The ideal astronomical telescope is, indeed, one which will show Sirius, or any other bright star, as nearly identical as possible with Euclid’s definition of a point, being that which has no parts and no magnitude. Lastly, what is Sirius made of? The late discovery of spectrum analysis, or of the instrument called the spectroscope, helps us here. A little further explanation on this subject will be given later. It may be observed in passing that before the said discovery, astronomers can scarcely be asserted to have known with any certainty that stars were suns. They could, indeed, be sure that at the vast distances of the fixed stars no bodies, shining by merely reflected light, could possibly be visible to us. But there knowledge stopped.