REPORTS FROM THE SECTIONS. 247 
Some Remarks on the Mounting of large 
Object-glasses. 
By H. C. Russexz, B.A., F.R.A.S. 
[Read before the Astronomical Society, on 1 November, 1878.] 
different pieces of glass, and mathematical formula all ready to 
his hand, determine exactly what curves his lenses should have in 
order to effect his purpose,—yet the exact accomplishment of these 
curves in each case is a matter of chance; he can be sure of 
getting very near what he wants, but to be exact is more than 
human. His only resource, therefore, is by trial to make the best 
of the lenses he has made, and his patience is, I believe, generally 
rewarded by the best result under the circumstances. When he 
as placed and marked the glasses, as a rule it is folly to attempt 
improvement ; yet there are exceptions in my own experience of 
so much practical importance that I have ventured to say a few 
words about them this evening. 
Some years since 1861 the Observatory received from Messrs. 
Merz and Sons, of Munich, a 7}-inch refractor, which appeared to be 
in every respect a first-class instrument. In 1870 I began to use 
m., and the focus is shortened about two inches, the original 
focal length being 10 feet 4 inches. The companion of Antares is 
now quite an easy object, and the definition, under favourable 
conditions of atmosphere, leaves nothing to be des 
power of 800 has been used with advantage. : Us 
uch experiments are troubl and not without risk, still it is 
baie while to make the experiment when mounting large lenses. 
eee ies ae 2 py er : 
on three points only, and in the 7} inch Mertz I could never detect 
any flexure from this cause, although the glass is not very thick. 
_ When I received the 11}-inch Schroeder telescope T had not 
the experience with Merz, and I tried sep 
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