THE MODERN EEFLECTING TELESCOPE. 11 



and is kept sci'aped clean l>y two scrapers which reach down into it from the 

 revolving turntable. Several important results are thus secured : the carpet cushion 

 under the glass is kept dry; the entire machine is kept pei'fectly free from the 

 drip[>ing of the grinding matei'ial ; and all of the latter material is caught in 

 buckets and is used ajjain and acrain in the later and finer grindins. 



The large irregularities of surface of large rough disks are usually ground 

 away witli coarse emery and a heavy, flat, half-size iron tool without grooves, the 

 surface of wdiich is rounded up considerably at the edge, so that the tool may rise 

 easily over obstructing iri'egularities without breaking them. The grinding machine 

 is set so that the half-size tool moves over the glass well out to one side of the lat- 

 ter ; the rotation of the turntable of course brings all parts of the glass in succession 

 under the tool; if the setting of the machine is such that the half-size tool passes 

 in much beyond the center of the glass at every stroke, the surface of the latter will 

 become concave. 



When the niai-ked irregularities of surface are ground away, the full-size, flat, 

 grooved iron tool is put on. A tool of this kind is almost indispensable in making 

 a mirror. Emery and water are supplied through the cups at the back of the tool, 

 and the glass is quickly ground aj)proximately flat. The glass is now turned over, 

 and the other side is ground in a precisely similar manner. 



The thickness of the glass is now tried, all around, by means of calipers. The 

 approximately flat surfaces will probably be found to be far fi'om parallel. If this 

 is the case, the thick side may be ground down as follows : The belt which drives 

 the turntable is loosened, until it will Just rotate the latter, and a brake is arranged 

 so that the workman can stop the rotation of the turntable at any <lesired point by 

 pi'essing on the brake with his foot. A flat, half-size grooved tool is put on, and 

 set so as to work far out to one side of the glass. A medium grade of emery (No. 

 70) is used, and the machine started. As the thick side of the glass, which has 

 been marked, comes beneath the moving tool, the tui-utable is slowed down or 

 stopped, so that a great excess of grinding is done on the thick side at each revolu- 

 tion. By distributing the grinding carefully, and trying the thickness often with 

 the calipers, the upper surface is easily made parallel to the lower one. When this 

 is done the full-size tool is again used for a short time. The glass is then ready for 

 edge-giinding. 



CHAPTER VII. 



GRINDING EDGK OF GL.\SS.— ROUNDING OF CORNERS. 



In order that an efficient edge-support, which will be described later, may be 

 fiven to the glass, it is desirable that the edge of the latter be ground truly circu- 

 lar and square with the face. The manner in which this is accomplished is shown 

 in Plate vi. The glass lies upon three large blocks of wood, which hold it several 



