THE MODERN REFLECTmG TELESCOPE. 5 



stroke; this change is especially desinible when figuring the glass; it is, of course, 

 impossible when only one driving-crank is used. 



I regard the transverse slide, or something equivalent to it, as absolutely neces- 

 sary to the success of a grinding and polishing macliine; it will be noticed that its 

 purpose corresponds, in some measure, to that of the long slot in the main arm of 

 Draper's machine; I have used both arrangements and have found tlie transverse 

 slide to be far more effective and convenient in use; its use will be described in 

 the chapters on grinding and polishing. 



The secondary crank, while very desirable and convenient, for the reason given 

 above, is not indispensable; 1 have used several smaller machines which have given 

 good results without it. 



The manner in which the grinding and polishing tools are connected to the 

 main arm is shown in Plate iv. A vertical shaft, 1| inches in diameter and 24 

 inches long, both rotates and slides (vertically) freely in bronze bearings attached to 

 the main arm. The grinding and polishing tools are connected to the lower end of 

 this shaft through the medium of a large universal coupling, — a gimball or Hooke's 

 joint, — with two pairs of horizontal pivots at right angles to each other ; this allows 

 the tools to rock freely in all directions in order to follow the curvature of the glass. 

 The tools are lifted, for counterpoising them, by the lever above (see Plate rv), 

 through the medium of the vertical shaft and the universal coupling. In the case 

 of veiy massive grinding tools of moderate size, like that shown in this illustration, 

 the universal coupling is connected directly to the back of the tool; but in the 

 case of all large tools which are to be used for fine work this connection is made 

 through the medium of a system of bars and triangles, so that the tools are counter- 

 poised without the slightest danger of changing their shape. A small coupling with 

 ball bearings at the upper end of the vertical shaft allows the latter to rotate freely 

 with reference to the link which connects it to the counterpoise lever. 



To recapitulate briefly: this method of connecting the grinding and polishing 

 tools allows them to be controlled in all of the following ways simultaneously : 

 (1) the stroke of the tool is given by the motion of the main arm ; '(2) the slow 

 rotation of the tool is rigoiously controlled by the lielting above; (3) the tool is 

 allowed to rouk or tip freely by means of the univei'sal cou[)ling, in ordei- that it 

 may follow the cuivature of the glass ; (4) the tool rises and falls freely by means 

 of the slidinf of the l|-inch vertical shaft in its bearings, in order that it may fol- 

 low the curvature of the glass; (5) the tool is counter^joised by means of the lever 

 on the main aim, thi-ouirh the medium of the same vertical shaft and universal 

 coupling. 



In Plate m is shown the laige lever by which the o-foot glass, which weighs 

 a ton, is lifted on and off the machine, and Ijy means of which, also, the hirge grind- 

 ing tools are handled. One of the full-size grinding tools, weighing 1,000 pounds, 

 is shown suspended by the lever. The ai-rangements are so convenient that the 

 optician alone can do all pai-ts of the work. 



