A SILVERED GLASS TELESOOPE. 



11 



the assistance of these templets, the convex tools of lead and iron and tin; concave 

 surface of the mirror are m:u\c parts of a sphere of proper diameter. 



The excavation of a large fiat disc of glass to a concave is best acconiplislied by 

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

 The central parts wear away very quickly, and w]\vu tliey become too flat must be 

 made convex again by striking the lead on the back with a hammer. The glass is 

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

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

 well by setting the conv(>x gauge on a diameter of the mirror, and observing how 

 many slips of paper of a definite^ thickness Avill ])ass under the centre or edge, as 

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

 pa])er is foimd correctly enough by measnrhig a half ream, and dividing by the 

 niiinbcr of sheets. In this manner differences in the versed sine of a thousandth 

 of an incli may be appreciated, and a close enough approximation to tlie desired 

 focal length reached — the precision required in ac^hromatics not being needed. 

 The preparation of the iron tools on which the grinding is to be finished is v(>ry 

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

 that they may be easily handled, and lience cannot be turned with very great 

 exactness. 



The pair for my larg(> mirrors are 151 inches in diameter, and were cast ^ of an 

 inch thick, being strengtlu>ned however on tlie back bj- eiglit ribs | of an inch high, 

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



Fig. 6. 



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'■"■Vj 



w 



The Iron Grinder. 



pounds apiece. Four ears, witli a tapped liole in eacli, ])roject at equal distances 

 round the edge, and serve either as a means of attachnuMit for a counterpoise lever, 

 or as liandles. 



After these were 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 w(M>k 

 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 tlu-y keep their focal length 

 and figure apparently witlu)ut change if carefully used, and only worked on glass 

 of nearly similar curvature. At first no grooves were cut )q)on tln^ face, for in tlie 



