LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
505 
first reflecting telescope, are preferable to any other with which I am acquainted ; 
the best proportions being four atoms of copper to one of tin (Turner’s numbers), in 
fact, 126 - 4 parts of copper to 58‘9 of tin. This alloy, however, as well as every speci- 
men of speculum metal I have ever examined, is visibly porous when carefully tested 
with a microscope; a very high power is not necessary: Coddington’s microscope, 
even with its lowest power, is generally sufficient ; with his highest power always so. 
When the copper is very impure, and the alloy has been much heated, then the pores 
are often easily perceptible to the naked eye : contrary, however, to what might have 
been inferred from this, by making the copper and tin chemically pure, we do not 
get rid of them altogether; pores can still be detected by the microscope, even when 
the metal has been melted at the lowest temperature. Good cast steel is free from 
pores, but cannot be hardened without cracking when of considerable dimensions : 
all copper is, I believe, porous, tin not so. 
Newton, with his usual acuteness, observed this porousness in speculum metal, and 
considered it a serious defect ; and, although, where the process of polishing is con- 
ducted in the best manner, the evil which he had apprehended, viz. the rounding of 
the edges of the cavities, is not perceptible, still it is probable that it always takes 
place in some degree, however trifling, producing a defect so mixed up with other 
unavoidable errors as to remain undetected. The smallest defect, however, deserves 
attention, and perhaps others may succeed in discovering the cause of this, which I 
have not : my experiments lead me to think it is in some way connected with the 
presence of carbon or oxygen ; and that possibly by watching the toughening of 
copper on a great scale, a process which chemistry has not satisfactorily explained, 
some information might be obtained. 
Upon the whole, therefore, there is at present no sufficient reason for making use of 
copper in any other state than that in which it can be procured from the merchant, 
except perhaps for the small flat metals of the Newtonian ; and with block-tin in the 
proper proportions, we have upon the whole the best material yet discovered for the 
specula of reflecting telescopes. It is, however, an alloy which has always been 
found rather difficult to manage ; it is not easily cast sound, even of moderate dimen- 
sions, by the ordinary processes of the founder, or by those which have been published 
by scientific men ; and even when so cast is very liable to break in cooling, or after- 
wards by exposure to any slight but sudden change of temperature, sometimes break- 
ing even without any apparent cause. 
The difficulties appeared so great, that reflecting upon Smeaton’s account of Sir 
W. Herschel’s labours, I had very little hope that any very large specula could be 
cast and safely polished of so brittle and untractable a material ; and although some 
of the difficulties have been surmounted, still it is probable, that in seeking to obtain 
the utmost possible amount of telescopic power the state of our atmosphere admits 
of, the requisite dimensions will not be attained by a simple casting of this alloy. 
The idea, therefore, obviously suggested itself of uniting several castings into one re- 
MDCCCXL. 3 T 
