44 THE MODERN REFLECTING TELESCOPE. 



other, which lie in a plane parallel to the back of the mirror. One of these levers 

 is shown in elevation at c, Fig. 17, and in plan in Fig. 21. The positions of the 

 nine levers are indicated by dotted crosses in Figs, 16 and 18. These levers are 

 suspended between pivots screwed thi-ough lugs connected to the cell. The cone 

 bearings, shown in Fig. 21, are finely fitted, and are ground to reduce friction. The 

 long arms of the levels carry adjustable lead weights (^/ Figs. 17 and 21) which are 

 made in the form of plates, in order that they may occupy as little space as possible 

 perpendicular to the plane of the miiror ; the short arms of the levers are thus made 

 to press against the backs of the coi'resjtondiTig segments thiough the medium of 

 light plates of bronze re[)resented by the single circles b Fig. 16 and at b Figs. 17 

 and 21. 



The large mirror weighs very neai'ly 2000 pounds, so that each segment 

 weiglis 166f pounds. With the cell in a horizontal position the lead weight on 

 each arm is adjusted until it just balances a standard weight of 166| pounds placed 

 upon the plate on the short arm. This adjustment being completed the miiror is 

 laid upon the support-system ; three-quarters of its weight is carried by the nine 

 levers, leaving one quarter to be divided equally between the three heavy plates a. 

 Thus each of the twelve segments is entirely supported at the back, independently 

 of all of the other segments. Now suppose that the edge-support, which will be 

 described below, be introduced, and the entire system, with the glass, inclined in 

 any direction and at any. angle; all of the levers and weights retain the same posi- 

 tion as before with refei'ence to the glass, but they do not exert the same pressure, 

 on account of the inclination ; so far as the back-support is concerned there will 

 still be a perfect balance maintained in the case of each segment; this is true what- 

 ever point of the edge of the mirror becomes lowest — i. e., in whatever direction 

 the levers lie with respect to tlie vertical plane through the axis of figure of 

 the mirror. 



It should be noticed that in the case of each of the twelve 10-inch supporting 

 plates only a ring one inch wide around the edge is in contact with the glass; the 

 part of each plate inside of this ring consists of deep, narrow arms, which do not 

 touch the glass, and which allows free access of air to the latter. 



Foi- very large or thin miri'ors a larger numbei- of plates and level's can 

 of course be used. An incidental advantage which occurs when this is done 

 is that the base of stable support afforded by the three stationaiy plates is still 

 larger, compared with the size of the mirror, than when twelve plates are used. 



U.—The Edge-Support. 



The relation between the back-support and edge-support is so intimate that any 

 inefficiency in the latter must injuriously affect the operation of the former, how- 

 ever perfect that may l)e in itself. In an equatorial reflecting telescope, different 

 parts of the edge of the mirror become successively lowest, as the position of the 

 telescope changes. With the flexible band and cushioned edge-support so much 

 used in the past, the heavy mirror necessarily changes its position, laterally, with 



