504 Penetration into Space by Telefcopes. 



The boundaries of vlfion, however, are not confined to fingle ftars. Where the light of 

 thefe falls fliort, the united luftre of fidereal fyftems will ftill be perceived. In clear nights, 

 for inftance, we may fee a whitifli patch in the fword-handle of Perfeus*, which contains 

 fmall ftars of various fizcs, as may be afcertained by a telefcope of a moderate power of 

 penetrating into fpace. We eafily fee the united luftre of them, though the light of no 

 one of the fingle ftars could have affefted the unaflifted eye. 



Confiderably beyond the diftance of the former muft be the clufter dlfcovered by Mr. 

 Meflier, in 1764 ; north following H. Gemlnorum. It contains ftars much fmaller than 

 thofe of the former clufter ; and a telefcope (hould have a confiderable penetrating power, 

 to afcertain their brightnefs properly, fuch as my common 10- feet refledbor. The night 

 fliould be clear, in order to fee it well with the naked eye, and it will then appear in the 

 fhape of a fmall nebula. 



Still farther from us muft be the nebula between ^ and K Herculis, difcovered by Dr. 

 Halley, in 1714. The ftars of it are fo fmall that it has been called a nebulaf ; and has- 

 been regarded as fuch, till my inftruments of high penetrating powers were applied to it^ 

 It requires a very clear night, and the abfence of the moon, to fee it with the natural aye. 



Perhaps, among the fartheft objefts that can make an impreflion on the eye, when not 

 aflifted by telefcopes, may be reckoned the nebula in the girdle of Andromeda, difcovered 

 by Simon Marius, in 1612. It is however not difficult to perceive it, in a clear night, on 

 account of its great extent. 



From the powers of penetrating into fpace by natutal vifion, we proceed now to that of 

 telefcopes. 



It has been ftiewn, that brightnefs, or light, is to the naked eye truly reprefented by 



yr ; in a telefcope, therefore, the light admitted will be exprefled by -^. Hence it 



would follow, that the artificial power of penetrating into fpace fhould be to the natural 

 one as A to a. But this proportion muft be corredled by the pradical deficiency in light 

 refle£l;ed by mirrors, or tranfmitted through glafies ; and it will in a great meafure depend 

 on the circumftances of the workmanfliip,. materials, and conftrudion of the telefcope, how 

 much lofs of light there will be fuftained. 



In order to come to fome determination on this fubjeft, I made many experiments with 

 plain mirrors, poliflied like my large ones, and of the fame compofition of metal. The 

 method I purfucd was that propofed by Mr. Bouguer, in his Traite d'Optique, page 16^ 



* See the catalogue of a fecond thoufand of new nebulae and clufters of ftars, VI. 33, 34., Phil. TranC 

 Vol. LXXIX. page 151. 

 + In the Conmilfance des temps for 178 J, No. 13, it is defcribed as a nebula without ftars. 



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