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it is notorious that even small reflectors act very imperfectly : 

 and he was therefore unprepared for any tolerable action of 

 this gigantic speculum. In the day time it was of course 

 colder than the air, and, if uncovered before that had sunk to 

 its temperature, was covered with dew : when this went off" it 

 always defined sharply. The huge mass of metal cooled much 

 more slowly than the atmosphere ; and as the difi'erence in- 

 creased, the performance of the telescope was deteriorated. 

 This arose from no change of figure, as he satisfied himself 

 by throwing the stars out of focus ; it was probably the result 

 of currents in the tube occasioned by this difference of tempe- 

 rature. How far it will be possible to obviate this by mecha- 

 nical means, remains to be tried ; but it is certain that the in- 

 convenience does not increase in a higher ratio than the power 

 of the telescope, as he had formerly apprehended. On the 

 same nights, it defined quite as well as the three-feet with a 

 far lower power ; and therefore, it is reasonable to expect that 

 it can be used with advantage much more frequently than he 

 once supposed. 



Enormous as is its illuminating power, it might be in- 

 creased one-third, by using it with the front view, supposing 

 it can be properly figured for this oblique action. Without 

 that, he fears that in an instrument where the aperture is so 

 large compared with the focal length, the definition would be 

 imperfect. He verified this by an experiment with the 

 three feet, and found that though the light was increased 

 quite as much as he expected, yet the perfection of the image 

 was utterly destroyed for large stars. There was no exact 

 focus, but merely two places where the sections of the cone 

 of rays were smallest. One, the least exceptionable, shewed 

 a flare in the direction of the slope like a comet's tail : at the 

 other this disappeared, but the star became a sort of curved 

 rectangle with rays from its corners. In the Newtonian 

 form this speculum a few nights before had defined ^ Ononis 

 very well with 500 ; but now y Leonis could not be seen 



