5q6 philosophical transactions. [anno 1800. 



the nebula north preceding Flamsteed's 1 trianguli, discovered by Mr. Messier, in 

 1764. " 7-feet reflector ; power 57. There is a suspicion that the nebula con- 

 sists of exceedingly small stars. With this low power it has a nebulous appear- 

 ance ; and it vanishes when I put on the higher magnifying powers of 278 and 

 460." — Oct. 28, 1794, I viewed the same nebula with a 7-feet reflector. " It is 

 large, but very faint. With 120 it seems to be composed 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 on 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 mag- 

 nified 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. June 18, 1799> I viewed the planet Venus 

 with a 10-feet reflector. " Its light is so vivid that it does not require, nor will it 

 bear, a penetrating power of 29, neither with a low nor with a high magnifying power." 

 This is not owing to the least imperfection in the mirror, which is truly parabolical, 

 and shows with all its aperture open, and a magnifying power of f)00, the double 

 star y Leonis in the greatest perfection. " It showed Venus perfectly well defined 

 with a penetrating power as low as 14, and a magnifying power of 400, or 600." 

 Here, penetrating power was injurious to magnifying power ; and that it necessarily 

 must be so, when carried to a high pitch, is evident ; for, by enlarging the aperture 

 of the telescope, we increase the evil that attends magnifying, which is, that we 

 cannot magnify the object without magnifying the medium. Now since the air is 

 very seldom of so homogeneous a disposition as to admit to be magnified highly, 

 it follows that we must meet with impurities and obstructions, in proportion to its 

 quantity. But the contents of the columns of air through which we look at the 

 heavens by telescopes, being of equal lengths, must be as their bases, that is, as 

 the squares of the apertures of the telescopes ; and this is in a much higher ratio 

 than that of the increase of the power of penetrating into space. From my long 

 experience in these matters, I am led to apprehend, that the highest power of mag- 



* 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 magnifying power m, the number of points n increases as the 



N 



squares of the power, the expression for the intrinsic brightness — , will decrease in the same ratio ; and 



/ 1 m 1 . 



it will consequent^ be in general n a m 1 , and — or c a — 3 that is, by compounding cn a — - = * = 1 5 



Km 2 "* 



or absolute brightness a given quantity. M. Bouguer has carefully distinguished intrinsic and absolute 

 brightness, when he speaks of the quantity of light reflected from a wall, at different distances. Traitc 

 d'Optique, page 39 and 40. — Orig. 



