It is evident that such an enormous instrument as this can not be made to travel by clockwork with the necessary uniformity without some very efficient arrangement for reducing friction. Dr. Common’s plan—and it is here that his instrument is unlike others—is to make the hollow polar axis watertight, and to fix it in a tank of water. At the bottom of the polar axis is a ball and socket joint to keep it in position, and at the top is another bearing, which can be adjusted so that the polar axis lies truly in the meridian. It was found necessary to introduce nine tons of iron into the bottom of the hollow polar axis in order to sink it to the proper angle, and to put sufficient weight on the bearings to give stability to the instrument. In this way the great mass is brought into the region of manageability, and the driving-clock, which is driven by a weight of one and a half tons, is able to do its work efficiently. Such, in general outline, is this wonderful telescope, which, although not so large as Lord Rosse’s famous instrument, is undoubtedly its superior in light-grasping power and general utility, and more especially in its adaptability for photographing the heavens. Among other large reflecting telescopes now in use are the 4-foot reflectors at Melbourne and Paris, and the 3-foot reflectors at South Kensington and the Lick Observatory, California. The largest refracting telescope yet constructed is one of forty inches aperture for the Yerkes Observatory of the University of Chicago. It is interesting to note here that Professor Keeler, in his report as an expert upon the performance of the object-glass, considers that there is “evidence for the first time that we are approaching the limit of size in the construction of great objectives.” Unlike a mirror, a lens can be supported only upon its circumference, and it is the bending by its own weight that proves detrimental to its defining power. If the lens be made thicker with a view of overcoming this defect, the absorption of light by the glass increases, so that there is in the end no special gain by increasing the size.