Pendration into Space by Telefcopa. . 553 



Sept. 20, 1783, I viewed the nebula between Flamfteed's 99th and 105th Pifclum, dif- 

 coveredby Mr. Mechain, in 1780. 



" It is not vifible in the finder of my 7-feet telefcope ; but that of my 20- feet (hews it." 



0£t. 28, 1784, I viewed the fame obje£t with the 7-feet telefcope. 



" It is extremely faint. With a magnifying power of 120, It feems to be a colIe(flion of 

 ** very fmall ftars : I fee many of them." 



At the time of thefe obfervations, my 7-feet telefcope had only a common finder, with . 

 an aperture of the objedl glafs of about ^ of an inch in diameter, and a fingle eye-len^ ; 



therefore its penetrating power was — l-2?_!ill5i_— 3,56, The finder of the 29-feet 



2 



inftrumcnt, being achromatic, had an obje£t glafs 1,17 inch in diameter : its penetrating 



1 r- ■^ '85 X 11,7)* 



power, therefore, was ■ — iz: 4,50. 



Now, that one of them fhewed the nebula and not the other, can only be afcribed to 

 fpace-penetrating power, as both inftruments were equal in magnifying power, and that 

 fo low as not to require an achromatic objeft glafs to render the image fufiiciently diftinft. 



The 7-feet refleftor evidently reached the ftars of the nebula; but its penetrating and 

 magnifying powers are very confiderable, as will be fliewn prefently. 



July 30, 1783, I viewed the nebula fouth preceding Flamfteed's 24 Aquarii, difcovered 

 by Mr. Maraldi, in 1746. 



" In the {mall/weeper*, this nebula appears like a telefcopic coinct." 



0€t. 27, 1794. The fame nebula with a 7-feet refledlor. 



" I can fee that it is a clufter of ftars, many of them being vifible." 

 , If vje compare the penetrating power of the two inftruments, we find that we have In 



thefirft"^'^'^^^'"'^^ = 12,84; and in the latter ^^ ^^3 x 63'- 12- _ 

 2 2 



However, the magnifying power was partly concerned in this inftance ; for, in the/weeper 



It was not fufficient to feparate the ftars properly. 



March 4, 1783. With a 7-feet refleftor, I viewed the nebula near the 5th Serpentis, 



difcovered by Mr. Meflier, in 1 764. 



• The fmall fweeper is a Newtonian refleflor, of two feet focal length; and, with an aperture of 4,1 

 Inches, has only a magnifying power of 24, and a field of view 2° iz'. Its diftinilnefs is fo perfefl, that 

 it will (hew letters at a. moderate diftance, with a magnifying power of 2000 ; and its movements are fo 

 convenient, that the eye remains at reil while the inftrument makes a fweep from the horizon to the zenith. 



A large one of the fame conftruftion has an aperture of 9,2 inches, with a foca! length of 5 feet 3 inches. 

 It is alfo charged low enough for the eye to take in the whole optic pencil ; and its penetrating power. 



, . . v^ ,41 X 9^' — 2i« 

 with a double eye glafs, is n ■ ■ 1 ~ t8,57t 



Vox. iV!--Mar<:h 1801. 4B "It 



