The observation is very unlike those of which we have hitherto spoken. The object is not to ascertain the actual position in the sky of the two stars, but their relative position to each other. A spider's thread of the finest strands is moved from one star to the other by turning an exquisitely fine screw; this enables us to measure their distance apart. Another spider thread at right angles to the first is laid through the centres of both stars, and a divided circle enables us to read the angle which this line makes to the true east and west direction. Such observations repeated year after year on many stars have enabled the orbits of not a few to be laid down with remarkable precision; and we find that their movements are completely consistent with the law of gravitation. Further, just as Neptune was pre-recognized and discovered from noting the irregularities in the motion of Uranus, so the discordances in the place of Sirius led to the belief that it was attracted by a then unseen companion, whose position with respect to the brighter star was predicted and afterwards seen. [Illustration: THE SOUTH-EAST DOME WITH THE SHUTTER OPEN.] Gravitation thus appears, indeed, to be the Bond of the Universe, yet it leaves us with several weighty problems. The observation of the positions of stars shows that though we call them fixed they really have motions of their own. Of these motions, a great part consists of a drift away from one portion of the heavens towards a point diametrically opposite to it, a drift such as must be due, not to a true motion of the individual stars, but to a motion through space of our sun and its attendant system. The elder Herschel was the first to discover this mysterious solar motion. Sir George Airy and Mr. Edwin Dunkin, for forty-six years a member of the Greenwich staff, and from 1881-1884 the Chief Assistant, contributed important determinations of its direction. What is the cause of this motion, what is the law of this motion, is at present beyond our power to find out. Many years ago a German astronomer made the random suggestion that possibly we were revolving in an orbit round the Pleiades as a centre. The suggestion was entirely baseless, but unfortunately has found its way into many popular works, and still sometimes is brought forward as if it were one of the established truths of astronomy. We can at present only say that this solar motion is a mystery.