114 



ON LORD ROSSE's TELESCOPE. 



Dr. Robinson, when giving, in November, 1840, to the 

 Academy, an account of the three-feet telescope constructed 

 by the Earl of Rosse, had announced to them the intention of 

 that nobleman to attempt an instrument of double aperture 

 and focal length. The attempt had succeeded even beyond 

 expectation, and he hoped that a brief notice of its progress 

 and results would not be uninteresting; more especially as he 

 felt that the approbation with which they had received his 

 former communication, and the importance which they at- 

 tached to Lord Rosse's discoveries, had not been the least 

 powerful cause of the triumph which their countryman has 

 now achieved. 



The speculum was cast on the 13th of April, 1842, ac- 

 cording to the principles which had been so successfully ap- 

 plied to the smaller mirrors ; but with several changes of the 

 details, made necessary by the gigantic scale of the work. 



It is well known to all who have experimented on specula, 

 that the alloy must be formed in the first instance, and re- 

 melted for casting at a much lower heat: otherwise the mirror 

 is full of pores. The fusion must, in both cases, be effected 

 in covered crucibles, to preserve the definite proportions of 

 the alloy, which would be lowered by oxidation of its tin if 

 exposed to the draught of the furnace. It is also necessary 

 that the speculum be of uniform composition and superficial 

 density; and as it is impossible to fuse the requisite quantity 

 of metal for one of six feet in a single vessel, the different por- 

 tions must flow into the mould under circumstances as nearly 

 as possible identical. Much thought and many experiments 

 must have been expended before these conditions were so 

 completely fulfilled. The crucibles are, of course, cast iron ; 

 no earthen one being able to bear the pressure of such a mass 

 of fluid metal at so high a temperature. They are thirty 

 inches internal depth, and twenty-four diameter, weighing 



