them both at the same distance. A little reflection, however, soon convinces him that the one star, though shining at a vast distance from the other, may be so placed in a line drawn from our eye to the latter as to be nearly or altogether eclipsed by it. Sometimes these stars are so close that the two pass for one, till an improvement in the telescope separates the companions, and shows them to be distinct. Herschel had this experience, and one of the most singular instances of it is not yet thirty years old. The dog-star Sirius is among the best known stars in our southern skies. Its brightness is forty- to sixty-fold that of the sun, its distance is such that a flash of light from it takes perhaps ten years to reach our eyes, and its weight exceeds that of two of our suns. This vast and brilliant sun was found to indulge in vagaries which were, and some of which still are, the puzzle of astronomers. They could not see, and therefore did not know. But although they could not see, they could imagine what the unseen cause of these vagaries was: for "the eyes of the mind can supply the want of the most powerful telescopes, and lead to astronomical discoveries of the highest importance."[1] Another star in the neighbourhood of Sirius, the mathematicians said, is moving round him. They calculated its orbit, they told observers where to apprehend the disturber, but in vain. At last the eighteen-inch object-glass, made for the Chicago Observatory in the United States, was turned on Sirius by way of trial. Great was the surprise of the manufacturers when they saw that the mighty sun had a fainter but a very bulky companion in his company, and was seen in the direction pre-
- ↑ Arago, Biographies, etc., p. 224.
