THE MODERN KEFLECTING TELESCOPE. 13 



CHAPTER VIII. 

 FINE-GRINDING AND POLISHING THE BACK OF THE MIRROR. 



Before discussing tlie work of fine-grinding I shall desci-ibe briefly the making 

 of the fine grades of emery. T ne\'er buy finer grades than "flour." The latter 

 grade is used with the full-size flat grooved tool to give a modeiately fine surface 

 to the glass after the rough-grinding previously described has made the front and 

 back approximately flat and parallel. The residue of emery, fine ground glass, and 

 water, resulting from the grinding with flour emery, is caught in buckets, as pre- 

 viously described. This residue is mixed vvitli an abundance of water, in (for a 

 large mirror) three or four clean granite-ware buckets, which ai-e marked A. The 

 contents of these buckets are thoroughly stirred, and are allowed to settle for two 

 minutes; during this time all coarse particles will have settled to the bottom, and 

 •'' two-minute " emery and finer grades remain in suspension in the water. The 

 liquid is now quickly siphoned off, by means of a rubber tube, into other clean 

 grauite-ware buckets marked B, from which the handles have been removed. The 

 contents of the latter are allowed to settle for four minutes, when the greatei' part 

 of the liquid in each is carefully poured back into the buckets^. The contents of 

 the latter buckets are reserved. The sediment remaining in the buckets B is the 

 "two-minute" washed emery, with which the fine-grinding of the back is begun. 

 After the grinding with this grade is finished, the residue from this grinding is 

 mixed with what was reserved in the buckets A, the whole is stirred asrain and 

 allowed to settle for five minutes, the liquid is sijthoned off, and thus "five-minute 

 washed " emery is secured. In a similar manner emeries which have remained in 

 suspension in water for 12, 30, 60, 120, and 240 minutes are secured. In this way 

 the large quantities of the finer grades which are necessary for large work can be 

 secured as the woi'k progresses. If accumulations of residues from previous work 

 are available, some time will be saved by washing out all of the fine grades desired 

 before the fiue-ijriudino: is beo-un. 



o o o 



Plane and concave mirrors are finished approximately flat on the back, as this 

 form is most convenient for the application of the support-system. Fine-grinding 

 of the back is usually done with the full-size, flat grooved tool, as this works 

 rapidly. In this part of the work, in which the greatest refinement is not neces- 

 sary, it is my custom to use the fine grades of emery (when these have all been 

 prepared in advance) in succession, without stopping the machine or taking off the 

 tool between grades for the purpose of cleaning the tool and the glass. The emery 

 and water are supplied through the wooden cups at the back of the tool. 



For a 24-inch mirror and its full-size tool, strokes varying from 6 to 8 inches 

 in length ai'e used with the 2-, 5-, and 12-minute washed emeries; shorter strokes, 

 from 4 to 6 inches in length, are used with the finer grades. Considerable lateral 

 displacement of the tool, amounting at the greatest to 2 or 2i inches on the glass, is 

 given at short intervals, by means of the transverse slide. On an average 20 double 

 strokes per minute are given in fine-grinding a 24-iuch mirror with full-size tool. 

 Between 7 and 8 double strokes occur for each revolution of the glass and turntable. 



