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double with any power; the companion of Rigel (some way 
from the meridian however) was lost in the flare, and even 
that of Polaris, though perfectly visible, was sadly disfigured 
by it. It was of course useless to try more difficult tests, as 
even this degree of imperfection would make it utterly incapa- 
ble of resolving such objects as the nuclei of the long nebule, 
be its illuminating power what it may. One thing, however, 
deserves notice, that in consequence of removing the second 
reflection, the colours of the stars come out with extraordinary 
splendour.* (3 Cygni, for instance, had a pureness and bril- 
lianey of yellow, in the large star, which was new to him, 
though he had seen it in many first-rate telescopes. Lord 
Rosse does not apprehend any insurmountable difficulty in 
applying his method to give the form necessary for aplanatic 
oblique reflection: more than one plan for this has occurred 
to him; and Dr. R. believes it is his purpose, as soon as the 
six-feet has its machinery completed, to try them on one of 
the three-feet specula, and, if successful, to alter the great 
one. 
As it is, Dr. R. congratulates the Academy and their 
country on the success of this matchless instrument; to which, 
as nothing at all approaching to its power has yet existed, so 
it is not probable that there will soon be any superior. It has 
been reported that the French Government, at the suggestion 
of M. Arago, are about to construct an achromatic of a metre 
aperture. Supposing homogeneous discs of glass can be obtained 
and wrought of that magnitude, there remain other difficulties. 
The optician who proposed tosupply them stated that they 
would weigh at least four hundred pounds; now these, when 
mounted, must be supported by at most two lateral bearings ; 
and it is known that a very moderate pressure produces in 
* The lenses of achromatics have often a tinge of green or straw colour 
which modify the colour of objects seen through them. Something of this 
may perhaps cause the predominance of green and ‘‘cinereus” which exists 
in the Dorpat catalogue. 
