All the bearings are constructed on the same principle as the Y bearings of a theodolite—that is, the pivots rest on two small portions of their arc, 90° or 100° apart. If allowed to rest on these bearings without some anti-frictional apparatus, the force required to move such an instrument would render it simply unmanageable and destroy the bearings. The plan adopted by Mr. Grubb is to allow the axis to rest in its bearings with just a sufficient portion of its weight to insure perfect contact, and to support the remainder by some anti-frictional apparatus. Generally 1/50 to 1/100 of the weight is quite sufficient to allow the axis to take its bearing, and the remainder 49/50 to 99/100 can thus be supported on friction rollers, and reduced to any desired extent, without injuring in the slightest degree the perfection of steadiness obtained by the use of the Y’s. This is the plan used in the bearings of the polar axis, and the result is that the instrument can be turned round this axis by a force of 5 pounds at a leverage of 20 feet. The bearings of the declination axis are supported on virtually the same principle; but the details of that construction are necessarily much more complicated, on account of the variability of direction of the resolved forces with respect to the axis. We may now turn to the four-foot silver-on-glass Newtonian now in course of completion at the Paris Observatory. The illustration which we give represents the telescope in a position for observation. The wheeled hut under which it usually stands, a sort of waggon seven metres high by nine long and five broad, is pushed back towards the north along double rails. The observing staircase has been fitted to a second system of rails, which permits it to circulate all round the foot of the telescope, at the same time that it can turn upon itself, for the purpose of placing the observer, standing either on the steps or on the upper balcony, within reach of the eyepiece. This eyepiece itself may be turned round the end of the telescope into whatever position is most easily accessible to the observer. [Illustration: FIG. 145.—Great Silver-on-Glass Reflector at the Paris Observatory. ] The tube of the telescope, 7·30 metres in length, consists of a central cylinder, to the extremities of which are fastened two tubes three metres long, consisting of four rings of wrought-iron holding together twelve longitudinal bars also of iron. The whole is lined with small sheets of steel plate. The total weight is about 2,400 kilogrammes. At the lower extremity is fixed the cell which holds the mirror; at the other end a circle, movable on the open mouth of the telescope, carries at its centre a plane mirror, which throws to the side the cone of rays reflected by the great mirror. The weight of the mirror in its barrel is about 800 kilogrammes; the eyepiece and its accessories have the same weight.[17]