A SILYKltKD (SLASS TKLKSCOPE. H 



tin- assistance of these templets, tin- convex tnols of lead and iron and the conc;i\c 

 surface of the mirror arc made parts of a sphere of proper diamcti r. 



The excavation of a large Hat disc- of glass to a concave is best accomplished by 

 means of a thick plate of lead, cast considerably more convex than the gauge. 

 The central parts wear awa\ very quickly, and when they liecomc- too Hat must be 

 made coin ex again hv striking the lead on the hack with a hammer. The glass is 

 thus caused gradually to approach the right concavity. Ten or twelve hours usiially 

 suttice to complete this stage. The progress of the excavating is tested sufficiently 

 well hy setting the convex gauge on a diameter of the mirror, and observing how 

 many slips of paper of a definite thickness will pass under the centre or edge, ag 

 the case may he. This avoids the necessity of a spherometer. The thickness of 

 paper is found correct!} enough by measuring a half ream, and dividing by the 

 number of sheets. In this manner differences in t lie \ersed sine of a thousandth 

 of an inch may he appreciated, and a close- enough approximation to the- desired 

 focal length reached--the precision required in achn.matics not being needed. 

 The preparation of the iron tools on which tin- grinding is to be finished is very 

 laborious where personal exertion is used. They require 1 to be cast thin in order 

 that they may be easilv handled, and hence cannot be turned with very great 



exactness. 



The pair for my large mirrors an- \~>\ inches in diameter, ami were cast $ of an 

 inch thick, being strengthened however on the back by eight ribs ^ of an inch high, 

 radiating from a solid centre two inches in diameter (, Fig. 6). They weighed 25 



Fig. 6. 



The Iron Grinder. 



pounds apiece. Four ears, with a tapped hole in each, project at equal distances 

 round the edge, and serve cither as a means of attachment for a counterpoise lever, 

 or as handles. 



After these WCTC turned and taken off the lathe chuck, they were found to be 

 somewhat sprung, and had to be scraped and ground in the machine for a week 

 before fitting properly. The slowness in grinding results from the emery becoming 

 imbedded in the iron, and forming a surface as hard as adamant. 



Once acquired, such grinders are very valuable, as they keep their focal length 

 and figure apparently without change if carefully used, and only worked on glass 

 of nearly similar curvature. At first no grooves were cut upon the face, for in the 



