A SILVERED GLASS TELESCOPE. 



35 



Fig. 32. 



softness of motion. Tho lower end of the frame was prevented from swinging back 

 vnd forward by a ntviA \m\ d, which phiyed along the glass rod e. All these ])arts 

 were attached to a frame /.', fitting on the eyepiece 

 hiildcr, and pcriiiitting tli(- rod <i^ to change from 

 the liori/.ontal jxisitioii in which it is here- drawn, to 

 any angular one desired. Tlie thuiid)-screw/' re- 

 tained it in place; ij and (/' are pulleys which per- 

 mit the cord to change direction. 



Subsetpieutly, a better method of examining the 

 uniformity of the rate, than l)y noticing the sharp- 

 n(>ss of the ])hotograph produced, was invented. It 

 consists in arranging a hxed microscope, magnify- 

 ing about 40 times, at the hack of the ground glass 

 plate, whicji fits in the same slide as the sensitive 

 plate. By watching the graiuilated appearance 

 pass before the eye, as the slide is moved by the 

 clock, the slightest variation from uniformity, any pulsatile or jerking movement is 

 rendered visil)l(>. By the aid of tliis microscopic exaggeration, it was seen that 

 occasionally, when there had been considerable changes in temperature, the steadi- 

 ness of the motion varied. Tliis was traced to the irregular slipping of 6, b'. 



A different arrangement was then ado])t(^d, by which a lunar crater can be 

 kept bisected as long as is necessary, and \\ liich gives origin to no irregularities, but 

 pursues a steady course. The principle is, not to allow a slipping friction anywhere, 

 but to substitute rolling friction, ni)()n wheels turning on points at the ends of their 

 axles. The following wood-cut is half the real size of this arrangement. 



Slidins' Plate-holder. 



Fig. 33. 



Frictionless Slide (front view). 



Sectional view. 



