16 THE JfODERN REFLECTING TELESCOPE. 



dust from the belts which control the slow rotation of the tools can fall upon 

 the glass. 



The process of grinding larger concave siirfaces without the use of full-size 

 tools is precisely similar to that described for a 24-inch luirror, up to the point of 

 substituting the 24-iuch convex tool ; from this point the grinding is carried on by 

 a continuation of the use of a half-size, convex grooved tool ; this may be the same 

 iron tool which has been used for enlarging the excavation. When the diameter 

 of the excavation approaches that of the glas.s, the tool should be tested with the 

 spherometer for curvature, and, if necessary, ground in its corresponding concave 

 iron tool until its curvature is uniform and of exactly the desired radius. The 

 grinding of the glass is then continued with washed flour emery until the edge of 

 the excavation is within ^ inch of tlie edge. Experience in the previous use of the 

 half-size tool, in enlarging, the excavation and in keeping the cui'vuture of the glass 

 uniform and of the desired radius, will enable the optician to decide upon the vari- 

 ous lengths of stroke and the various settings of the transverse slide necessary in 

 this orrindino; and in the finer crrindincr to follow. 



In fine-grinding a 80-inch concave with a 16-inch tool, strokes varying from 6 

 to 12 inches in length are used ; for a 9-inch stroke the normal setting of the trans- 

 verse slide (/. e., one which would tend neithei- to lengthen nor shorten the radius of 

 curvature of the glass) would be such that the outer edge of the tool overhangs the 

 glass about 3 inches in the forward stroke, while the inner edge of the tool passes 

 about one inch on the other side of the center of the glass on the return stroke. 



Throughout the entire process of fine-grinding with the half-size tool the length 

 of stroke is changed once every eight or ten minutes ; at the end of every two or 

 three revolutions of the glass the setting of the transverse slide is changed, a little 

 at a time, for a considerable distance on either side of the normal setting ; the set- 

 ting of the slide can be changed without difficulty, while the machine is running, by 

 merely turning a hand-wheel. By these means the formation of zones of unequal 

 focal length can be entii'ely avoided. 



The same grades of emery are used, and the same rules in regard to counter- 

 poising observed, as with full-size tools. Notwithstanding the fact that the length 

 of stroke can be considerably greatei" than with full-size tools, each gi-ade of emery 

 must be used for a lonfjei" time, on account of the smaller area of the fri'indino- sur- 

 face. Glass and tool are thoroughly cleaned, and the surface of the formei' exam- 

 ined, after the use of each grade of emery, as before desci'ibed. 



Care must be taken during this work that the belts which rotate the turntable 

 are kept tight, so that no iri-egularity in the rotation of the turntable with refer- 

 ence to that of the crank-shaft can occur. It is absolutely necessary that all of the 

 fine work on large mirrors be done in rooms where no sudden changes of tempera- 

 ture can occur, and that nothing be allowed which might affect the temperature of 

 the glass locally. 



If the concave mirror is intended for a paraboloidal one, the fine-ground surface 



T>-3 



should be spherical, with its radius of curvature 2 F +— =r, where F is the desired 



4r 



