12 THE MODERN REFLECTING TELESCOPE. 



inches above the surface of the circular iron pLite. Thin oilcloth is arranged 

 about the blocks and over the iron plate, to keep them dry. A smooth, flat, iron 

 face-plate is mounted (so as to rotate in a vertical plane) on a heavy lathe head- 

 stock; the latter is carried upon a strong slide which can be moved toward, the 

 glass by means of a fine pushing-screw. The lathe and face-plate are driven at a 

 hich rate of speed by means of a belt. In the case of the 5-foot glass the face- 

 plate used was 24 inches in diameter and made 1,000 revolutions per minute. For 

 a 24-inch glass, 3^ inches thick, a face-plate 11 inches in diameter, making 1,800 

 revolutions per minute, is used. A frame of wood, covered with oilcloth, is built 

 around the face-plate, so that the grinding materials will not be thrown about the 

 room. The glass i-otates slowly with the turntable, as usual. Emery and water, 

 or sand and water, are heaped upon the hoi'izontal surface of the glass, and ai'e 

 slowly sci-aped toward and over the edge, so as to come between the revolving face- 

 plate and the glass; a small jet of cold water, brought from the hydrant by means 

 of a rubber tube, greatly assists in the uniform feeding of the emery, and also in 

 preventing the generation of heat. But there is in reality no danger of heating, for 

 the revolving face-plate never actually touches the glass. As the iiregulaj'ities of the 

 edge are ground away the face-plate is gradually moved forward by means of 

 the slide and pushing-screw. 



If the edge of the rough disk be v'ery irregular, as is usually the case, the sur- 

 face of the iron face-plate will have a circulai- groove worn in it, by the time the 

 rough-gi-inding of the edge is done ; in this case the face-plate should be turned flat 

 and true again, and smoothed on a flat iron grinding-tool, before the edge of the glass 

 is fine-ground. Several fine grades of emery are now pre[>ared by the process of 

 washing to be described later, and the edge-grinding is finished by the use, in suc- 

 cession, of thi'ee such grades of emery as flour, three-minute washed, and ten-minute 

 washed. Care should be taken throughout the process that the edge of the glass 

 is ground square with the face ; any error in this respect can be corrected by slightly 

 raising or lowering the outer end of the slide which supports the lathe head-stock. 



Edge-grinding is accomjdished very quickly in the manner described. The 

 edge of a 24-iuch disk four inches thick, even when very rough and irregular, has 

 been ground and smoothed in ten hours of actual grinding. Despite the great 

 speed of the rotating face-plate, I have never had any chip[)ing of the glass or 

 accident of any kind occur. 



Before beginning the fine-grinding of the face and back it is well to round the 

 corners at the edge of the glass. This is done by means of a smooth flat strip of 

 sheet-brass of the size and shape of a large flat file ; this is worked over the corners 

 of the glass by hand, while the disk rotates slowly, emery and water being used for 

 cutting. A "quarter-round" corner is usually made. Finer and finer grades of 

 emery are used for smoothing the quarter-round. This rounding and smoothing 

 are very necessary, as paiticles of glass fi'om a shai'[) or I'ough edge are liable to be 

 drawn in upon the surface by the action of the grinding tool during fine-grinding. 



The wooden blocks are now removed and the glass replaced upon the carpeted 

 turntable. 



