penetrating into Space by Telescopes. 65 



67262 were returned ; and therefore, if a double reflection takes 

 place, only 45242 will be returned. 



Before this light can reach the eye, it will suffer some loss in 

 passing through the eye glass ; and the amount of this I ascer- 

 tained, by taking a highly polished plain glass, of nearly the 

 usual thickness of optical glasses of small focal lengths. Then, 

 by the method of the same author, page 21, fig. 5. 1 found, that 

 out of 100 thousand incident rays, 94825 were transmitted 

 through the glass. Hence, if two lenses be used, 89918 ; and, 

 with three lenses, 852% rays will be transmitted to the eye. 



Then, by compounding, we shall have, in a telescope of my 

 construction with one reflection, 63796 rays, out of 100 thou- 

 sand, come to the eye. In the Newtonian form, with a single 

 eye lens, 42901; and, with a double eyeglass 40681 will re- 

 main for vision. 



There must always remain a considerable uncertainty in the 

 quantities here assigned ; as a newly polished mirror, or one in 

 high preservation, will give more light than another that has 

 not those advantages. The quality of metal also will make some 

 difference; but, if it should appear by experiments, that the 

 metals or glasses in use will yield more or less light than here 

 assigned, it is to be understood that the corrections must be 

 made accordingly. 



We proceed now to find a proper expression for the power 

 of penetrating into space, that we may be enabled to compare 

 its effects, in different telescopes, with that of the natural eye. 



Since then the brightness of luminous objects is inversely as 

 the squares of the distances, it follows, that the penetrating 

 power must be as the square roots of the light received by the 

 eye. 



MDCCC. K 



