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 
