THE MODERN REFLECTING TELESCOPE. 15 



The glass must also be carefiill}' levelled (by means of the three large adjusting 

 screws of the turntable) so tliat its upper surface is accurately at right angles to 

 the axis of rotation ; this is determined by rotating the turntable, and trying the 

 surface with a surface-gauge. The band of thin oilcloth is securely bound around 

 the edge of the glass, to keep the polished back and the cushion clean and dry. 



The excavation of the concave is begun with moderately coarse emery (if the 

 concave is to be quite deep) and a lead tool ; this is a lead disk about 10 inches in 

 diameter and 1^ or 2 inches thick; it is easily turned in a lathe to the proper 

 curvature ; it is used on aud near the centei' of the glass until a depression of 

 approximately the desired curvature (as determined by the spherometer) and of 12 

 or 13 inches diameter is produced. A heavy iron tool about 13 inches in diameter, 

 which has been turned and ground to the proper cui'vatuie, is now put on with 

 about No. 90 emery. By giving careful attention to tlie length of stroke, and to 

 the position of the tool on the glass as determined by the setting of the transverse 

 slide, and by frequent trials of the curvature of the excavation with the spherom- 

 eter, the diameter of the excavation is gradually increased, while its ciirvafure is 

 continuall}' kej)t veiy near that wiiich is desired for the Hnished miri'or; this keeps 

 the ii'on tool of proper curvature also. 



The stroke used in this work sliould vary from 6 to 10 inches in length. As 

 the size of the excavation increases, the setting of the transverse slide is continually 

 changed so that the tool acts farther and farther to one side of the center of the 

 glass ; otherwise the i-adiiis of curvature will be shortened. When the diameter of 

 the excavation has increased to about 22 inches, flour emery is substituted for the 

 No. 90, and the grinding is contiiuied as before. Care is nt)W taken to make the 

 curvature I'ead exactly I'ight with the spherometer. When the e.xcavation becomes 

 about 23 inches in diameter, the 13inch tool is taken off, and the full-size, convex, 

 grooved iron tool is put on ; this has previously been fine-ground to the proper 

 curvature on the corresponding concave, tool. With this tool aud washed flour 

 emery the diameter of the concave on the glass is increased to 23| or 23if inches. 



The fine-t;rindiu£c or smoothins; of the concave is now done with the full-size 

 tool. The same grades of emery, the same lengths and speed of stroke, and the same 

 rules in regard to counterpoising are used as have already been described in the 

 case of fine-grinding the back of the glass (page 13). The length of stroke is 

 changed every eight or ten uiiiuites, and the lateral displacement of the tool (given 

 by means of the transverse slide) is changed slightly at the end of every two or 

 three complete revolutions of the glass. The tool is taken off after each grade of 

 fine emery is used, and the tool and glass are carefully cleaned. With the assistance 

 of the counterpoise lever the removal of the tool is effected easily and safely, with- 

 out disconnecting it from the main arm of the machine ; this is well shown in Plate 

 IV, in which the grinding tool is shown hanging at one side of the glass. 



The surface of the glass is examined with a microscope after each grade 

 of emery is used, to make sui'e that no pits from previous grades remain. 



During all fine-grinding and raachine-pdlishing a large sheet of heavy clean 

 papei- or pasteboard is attached to the main arm in such a way that no particles of 



