£>4 Dr. Herschel on the Power of 



powers were applied to it. It requires a very clear night, and 

 the absence of the moon, to see it with the natural eye. 



Perhaps, among the farthest objects that can make an im- 

 pression on the eye, when not assisted by telescopes, may be 

 reckoned the nebula in the girdle of Andromeda, discovered by 

 Simon Marius, in 1612. It is however not difficult to per- 

 ceive it, in a clear night, on account of its great extent. 



From the powers of penetrating into space by natural vision, 

 we proceed now to that of telescopes. 



It has been shewn, that brightness, or light, is to the naked 



a 1 l 



eye truly represented by -^- ; in a telescope, therefore, the 



A* l 



light admitted will be expressed by -^-. Hence it would fol- 

 low, that the artificial power of penetrating into space should be 

 to the natural one as A to a. But this proportion must be cor- 

 rected by the practical deficiency in light reflected by mirrors, 

 or transmitted through glasses ; and it will in a great measure 

 depend on the circumstances of the workmanship, materials, 

 and construction of the telescope, how much loss of light there 

 will be sustained. 



In order to come to some determination on this subject, I 

 made many experiments with plain mirrors, polished like my 

 large ones, and of the same composition of metal. The method 

 I pursued was that proposed by Mr. Bouguer, in his Traite 

 d'Optique, page 16, fig. 3.; but I brought the mirror, during 

 the trial, as close to the line connecting the two objects as pos- 

 sible, in order to render the reflected rays nearly perpendicular. 



The result was, that out of 100 thousand incident rays, 



