14 THE MODERN REFLECTESTfr TELESCOPE. 



With res^ard to couutei-poising the tools dining fiue-griudiug, the followiug 

 statements may l)e made : My full-size iron tools for a 24-inch rairroi- -weigh about 

 150 pounds, or ^ pound for each square inch of area. Tliis weight, or even ^ 

 pound to the square inch, is not objectionable with emeries down to 5-minute or 

 10-minute washed; but when this weight is allowed with finer emeries, sci-atches 

 are liable to occur; indeed, with 30-minute washed and all finer grades they are 

 almost certain to occui-. The pressure on tlie glass is therefore decreased, by coun- 

 terpoising the tool, to approximately J- pound to the square inch for 12- to 20 minute 

 emeries, l pound per square inch for 30- to 60-minute emeries, and about ^\ pound 

 per squai'e inch for 120- and 240-miuute emeries, litis rule is followed, approxi- 

 mately, in all fine-gi'incUng, ivhether of bach or face. This obviates, to a gi-eat 

 extent, the dangei- of scratches in grinding, provided that thorough cleanliness is 

 practiced in the pi'eparation and use of the fine emeries. The apparatus V>y which 

 the counterpoising is effected has already been described (page 5). 



In fine-grinding a 24-incli glass, the 2minute and 5-minute emeries are used for 

 three-quarters of an hour each; the 12- and 30-minute emeries for one hour each, 

 and the 60-, 120-, and 240-minute emeries for one and one-half hours each. The 

 fine-ground surface resulting is so exquisitely smooth that it takes a full polish ver}- 

 readily. 



The back of the glass is now ready to be polished. This is done with a half- 

 size or two-thirds size polishing tool, which is moved about on the glass by the 

 action of the machine precisely as a half-size grinding-tool wouhl be. Optical rouge 

 and distilled water are used, instead of emery and water. The work of polishing 

 will be desciibed in detail later, in connection with the work of finishing the face 

 of the glass. 



It is an excellent plan to fine-grind and polish the front surface of a disk 

 also, approximately flat, as has been described for the back ; the ojitician is then 

 able to examine carefully the internal structure of the disk. Usually there is no 

 choice as to which side shall be used for the face of the mirror, but this can i-eadily 

 be determined when both sides are polished. Plate vri shows the 5-foot disk with 

 both sides ground and polished in this manner. 



CHAPTER IX. 

 GRINDING THE CONCAVE SURFACE. 



As before stated, it is my practice to use full-size grinding tools for concave 

 mirrors up to 24 or 30 inches in diameter. For larger concave mirrors half-size tools 

 are generally used. I shall first describe the grinding of a 24-inch concave. 



The glass must be carefully centered by means of the three adjustable arcs 

 attached to the supporting plate ; these arcs must not be screwed tightly against 

 the glass, lest the latter be strained ; several thicknesses of heavy drawing paper 

 are used between arcs and irlass. 



