A SILVERED GLASS TELESCOPE. 



35 



Fig. 32. 



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

 and forward by a steel pin cZ, which played along the glass rod e. All these parts 

 were attached to a frame k, fitting on the eyepiece 

 holder, and permitting the rod a to change from 

 the horizontal position in which it is here drawn, to 

 any angular one desired. The thumb-screw/ re- 

 tained it ill place ; g and g' are pulleys which per- 

 mit the cord to change direction. 



Subsequently, a better method of examining the 

 uniformity of the rate, than by noticing the sharp- 

 ness of the photograph produced, was invented. It 

 consists in arranging a fixed microscope, magnify- 

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

 plate, which fits in the same slide as the sensitive 

 plate. By watching the granulated 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 visible. By the aid of this microscopic exaggeration, it was seen that 

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

 ness of the motion varied. This was traced to the irregular slipping of &, V . 



A different arrangement was then adopted, by which a lunar crater can be 

 kept bisected as long as is necessary, and which 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, upon wheels turning on points at the ends of their 

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



Sliding Plate-liolder. 



Fig. 33. 



Frlctionless Slide (front view). 



Sectional view. 



