OF FLUID REFRACTING TELESCOPES. 
11 
his instrument and mine ; this is Messier’s twenty-second nebula. This 
object,, which in a good 3|-inch refractor has only the appearance of a white 
cloud,, I saw in the above instrument resolved into an immense number of 
brilliant small stars. In my telescope also it is resolved into apparently as 
great a number of stars, but the full power of the instrument is exerted, and 
still the resolution seems scarcely complete. In fact, my instrument appears 
to labour to effect what seems to be quite within the power of the other. 
I wish particularly to direct attention to this object and that of (3 Capricorni, 
because, where the same object can be seen without any very apparent differ- 
ence in two instruments, or where it can be only seen in one, the great test of 
comparison is lost ; but in those I have mentioned, the exact limit of power is 
defined. 
Amongst the objects I have examined in Sir James South’s telescope, and 
repeated in mine, for the purpose of comparing the defining powers of the two 
instruments, were the planets Jupiter and Mars. These were both more 
sharply defined in Sir James South’s than in my telescope, but the superiority 
was by no means so great as I had expected. I fortunately saw the shadow 
of Jupiter’s third satellite pass over the disc, on the 8th of August, at Kensing- 
ton, and it exhibited a fine black round spot, extremely well defined ; and on 
the 13th of the same month, I witnessed precisely the same phenomenon in 
mine, and, as far as the definition of the shadow was concerned, with an effect 
in which I could not distinguish an inferiority ; it had the appearance of a 
small black wafer on a sheet of white paper ; still, however, the edge of the 
planet was certainly sharper in the twenty-feet. Both evenings were amongst 
the finest this climate affords, and the powers employed as nearly as possible 
equal ; viz. about 260 and 450 in both instruments. We also, at Kensington, 
observed Mars with various powers from 260 to 1400, and it carried 1200 well; 
with this the white spot near its south pole was seen beautifully distinct, as 
also a long dark spot on its apparent eastern limb. The bright spot at its 
south pole I saw also remarkably well defined in my own telescope on the 
13th, and a dark spot on its disc very distinct; but it occupied the centre of 
its disc. The highest power however I used was 500, and it was probably 
best seen with 260. 
There can be no question of the superior defining power of the twenty -feet, 
c 2 
