penetrating into Space by Telescopes. 79 



" power it has a nebulous appearance ; and it vanishes when I 

 " put on the higher magnifying powers of 278 and 4,60." 



Oct. 28, 1794. I viewed the same nebula with a 7-feet 

 reflector. 



" It is large, but very faint. With 1 20, it seems to be com- 

 " posed of stars, and I think I see several of them ; but it will 

 " bear no magnifying power." 



In this experiment, magnifying power was evidently injurious 

 to penetrating power. I do not account for this upon the principle 

 that by magnifying we make an object less bright ; for, when 

 opticians have also demonstrated that brightness is diminished 

 by magnifying, it must again be understood as relating only to 

 the intrinsic brightness of the magnified picture; its absolute 

 brightness, which is the only one that concerns us at present, 

 must always remain the same.* The real explanation of the 

 fact, I take to be, that while the light collected is employed in 

 magnifying the object, it cannot be exerted in giving penetrating 

 power. 



• This may be proved thus. The mean intrinsic brightness, or rather illumination, 

 of a point of the picture on the retina, will be all the light that falls on the picture, 



divided by the number of its points; or C — •^-. Now, since with a greater magni- 

 fying power m, the number of points N increases as the squares of the power, the 

 expression for the intrinsic brightness ■^r, will decrease in the same ratio ; and it will 



consequently be in general N oc m 1 , and ^-orCa — — ; that is, by compounding 



*** * ~T"T = ' == ■ ' » or absolute brightness a given quantity. M. Bouguer has 



carefully distinguished intrinsic and absolute brightness, when he speaks of the quan- 

 tity of light reflected from a wall, at different distances. Traite d'Optique, page 39, 

 and 40. 



