132 



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 nebulae, 

 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- 

 liancy 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 diflSculty 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 to supply 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. 



