592 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



a considerable degree of magnifying as well as penetrating power, it always ap- 

 peared plainly, that the instrument which had the highest penetrating power resolved 

 them best, provided it had as much of the other power as was required for the 

 purpose. 



Sept. 20, 1783, I viewed the nebula between Flamsteed's 99th and 105th 

 Piscium, discovered by Mr. Mechain, in 1 780. " It is not visible in the finder 

 of my 7-feet telescope; but that of my 20-feet shows it." — Oct. 28, 1784, I 

 viewed the same object with the 7-feet telescope. " It is extremely faint. With 

 a magnifying power of 120, it seems to be a collection of very small stars: I see 

 many of them." At the time of these observations, my 7-feet telescope had 

 only a common finder, with an aperture of the object-glass of about -f- of an 

 inch in diameter, and a single eye-lens; therefore its penetrating power was 

 -jV-899 X 7.5 2 = 3.56. The finder of the 20-feet instrument, being achromatic, 

 had an object-glass 1.1 7 inch in diameter; its penetrating power therefore was 

 4V. 85 X 11.7 a .= 4.50. 



Now, that one of them showed the nebula and not the other, can only be 

 ascribed to space-penetrating power, as both instruments were equal in magnifying 

 power, and that so low as not to require an achromatic object-glass to render the 

 image sufficiently distinct. The 7-feet reflector evidently reached the stars of the 

 nebula; but its penetrating and magnifying powers are very considerable, as will be 

 shown presently. July 30, 1783, I viewed the nebula south preceding Flam- 

 steed's 24 Aquarii, discovered by Mr. Maraldi, in 1746. " In the small sweeper,* 

 this nebula appears like a telescopic comet." — Oct. 27, 1794, the same nebula 

 with a 7-feet reflector. " I can see that it is a cluster of stars, many of them 

 being visible." If we compare the penetrating power of the 2 instruments, we 

 find that we have in the first -JV(.41 X (42 2 — 12 2 )) = 12.84; and in the latter 

 4-v/(.43 X (63 2 — 12 2 )) = 20.25. However, the magnifying power was partly- 

 concerned in this instance; for, in the sweeper it was sufficient to separate the 

 stars properly. 



March 4, 1783, with a 7-feet reflector, I viewed the nebula near the 5th Ser- 

 pentis, discovered by Mr. Messier, in 1764. " It has several stars in it; they are 

 however so small that I can but just perceive some, and suspect others." 

 May 31, 1783, the same nebula with, a 10-feet reflector; penetrating power 

 4V(.43 X (89 2 — 16*)) sb 28.67. ""With a magnifying power of 250, it is all 

 resolved into stars: they are very close, and the appearance is beautiful. With 

 600, perfectly resolved. There is a considerable star not far from the middle; 



* The small sweeper is a Newtonian reflector, of 2 feet focal length ; and, with an aperture of 4.2 

 inches, has only a magnifying power of 24, and a field of view 2° 12'. Its distinctness is so perfect, 

 that it will show letters at a moderate distance, with a magnifying power of 2000 j and its movements 

 are so convenient, that the eye remains at rest while the instrument makes a sweep from the horizon to 

 the zenith. A large one of the same construction his an aperture of 9.2 inches, with a focal length of 

 5 feet 3 inches. It is also charged low enough for the eye to take in the whole optic pencil ; and its 

 penetrating power, with a double eye-gUss, \s £ V(-*l X (92 l — 21 2 )) = 28.57. — Orig. 



