5g 1 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



With me, there is not a doubt remaining ; but another person, in order to form 

 a judgment, ought previously to go through all the several gradations of nebulae 

 which I have resolved into stars." — May 25, 1791, I viewed the same nebula with 

 a 20-feet reflector of my construction, having a penetrating power of 4V-64 X 

 188 2 = 75.08. " With a magnifying power of 157, it appears extremely bright, 

 round, and easily resolvable. W r ith 300, I can see the stars. It resembles the 

 cluster of stars taken at l6 h 43 m 40 s *, which probably would put on the same 

 appearance as this, if it were at a distance half as far again as it is. ra 17 h 26 m 

 19 s ; pd 93° 10'." Here we may compare 2 observations ; one taken with the 

 penetrating power of 39, the other with 75 ; and, though the former instrument 

 had far the advantage in magnifying power, the latter certainly gave a more com- 

 plete view of the object. 



The 20-feet reflector having been changed from the Newtonian form to my pre- 

 sent one, I had a very striking instance of the great advantage of the increased 

 penetrating power, in the discovery of the Georgian satellites. The improvement, 

 by laying aside the small mirror, was from 6l to 75 ; and whereas the former was 

 not sufficient to reach these faint objects, the latter showed them perfectly well. 

 March 14, 1798, I viewed the Georgian planet with a new 25-feet reflector. Its 

 penetrating power is 4V.64 X 240 2 = 95.85 ; and having just before also viewed 

 it with my 20-feet instrument, I found that with an equal magnifying power of 

 300, the 25-feet telescope had considerably the advantage of the former. 



Feb. 24, 1786, I viewed the nebula near Flamsteed's 5th Serpentis, which has 

 been mentioned before, with my 20-feet reflector; magnifying power 157- " The 

 most beautiful extremely compressed cluster of small stars ; the greatest part of 

 them gathered together into one brilliant nucleus, evidently consisting of stars, 

 surrounded with many detached gathering stars of the same size and colour, ra 

 15 h 7 m 12 s ; pd 87° 8'." — May 27, 1791, I viewed the same object with my 40- 

 feet telescope; penetrating power -iV.04 x 480 2 = 191.69; magnifying power 

 370. " A beautiful cluster of stars. I counted about 200 of them. The middle 

 of it is so compressed that it is impossible to distinguish the stars." Here it ap- 

 pears, that the superior penetrating power of the 40-feet telescope enabled me even 

 to count the stars of this nebula. It is also to be noticed, that the object did not 

 strike me as uncommonly beautiful ; because, with much more than double the 

 penetrating, and also more than double the magnifying power, the stars could rjot 

 appear so compressed and small as in the 20-feet instrument : this very naturally 

 must give it more the resemblance of a coarser cluster of stars, such as I had been 

 in the habit of seeing frequently. 



The 40-feet telescope was originally intended to have been of the Newtonian 

 construction ; but, in the year 1787, when I was experimentally assured of the vast 



• The object referred to is No. 10, of the Connoissance des Temps for 1783, called " Nebuleuse 

 sans etoiles." My description of it is, " A very beautiful, and extremely compressed, cluster of stars : 

 the most compressed part about 3 or 4' in diameter, ra. l6 h 4<5 m 2' ; pd 93° 46'." — Orig. 



