penetrating into Space by Telescopes. 71 



At the time of these observations, my 7-feet telescope had 

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

 about -| of an inch in diameter, and a single eye-lens ; there- 

 fore its penetrating power was ' " X7,sl =3,56. The finder 



of the 20-feet instrument, being achromatic, had an object 

 glass 1,17 inch in diameter; its penetrating power, therefore, 



was 2_ L. = 4}5 o. 



Now, that one of them shewed the nebula and not the other, 

 can only be ascribed to space-penetrating power, as both instru- 

 ments were equal in magnifying power, and that so low as not 

 to require an achromatic object glass to render the image suffi- 

 ciently distinct. 



The 7-feet reflector evidently reached the stars of the nebula ; 

 but its penetrating and magnifying powers are very consi- 

 derable, as will be shewn 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. 



# The small sweeper is aNEWTONiAN 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 1 2'. 

 Its distinctness is so perfect, that it will shew letters at a moderate distance, with a 

 magnifying power of 2000 ; 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 has 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 glass, is 



x/,4iX9**-2i a _ _ 2g)57> 



