12 Penett-aiion into Space by Telefcopes. 



" The 3d fatelHte feems to be fmaller than it was the laft night but one. The 4th fa- 

 " tellite feems to be larger than it was the 19th. This telefcope fliews the fatellites not 

 " nearly fo well as the 40-feet. 



Here, the magnifying power being nearly alike, the fuperiority of the 40-feet telefcope 

 muft be afcribed to its penetrating power. 



The difFerent nature of the two powers above mentioned being thus evidently efta- 

 bliflied, I muft now remark, that, in fome refpe£ts, they even interfere with each other; 

 a few inftances of which I (hall give. 



Auguft 24, 1783. I viewed the nebula north preceding Flamfteed's i Trianguli, dif- 

 covered by Mr. Meflier, in 1764. 



" 7-feet refleQor; power 57. There is a fufpicion that the nebula confifts of exceed- 

 " ingly fmall ftars. With this low power it has a nebulous appearance j and it vaniflies 

 *' when I put on the higher magnifying powers of 278 and 460." 



0£t. a8, 1794. I viewed the fame nebula with a 7-feet refieflor. 



" It is large, but very faint.' With 120, it feems to be compofed of ftars, and I think I 

 " fee feveral 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 lefs 

 bright j for, when opticians have alfo demonftrated that brightnefs is diminiflicd by mag- 

 nifying, it muft again be underftood as relating only to the intrinftc brightnefs of the mag- 

 nified pldlure -, its abfolute brightnefs, which is the only one that concerns us at prefent, 

 muft always remain the fame *. The real explanation of the faft, I take to be, that while 

 the light collefted is employed in magnifying the objedl, it cannot be exerted in giving 

 penetrating power. 



June 18, 1799. I viewed the planet Venus with a lo-feet refleilor. 



" Its light is fo 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 may be proved thus. The mean intrinfic brightnefs, or rather illumination, of a point of the 

 piflure on the retina, vi\\\he all the light that falls on the pidure, di'vided by the number 0/ its foints; or 



n —. -—, Now, fmce with a greater magnifying power m, the number of points N increafes as the 

 — H 



fquares of the power, the exprefliun for the intrinfic brightnefs — , will decceafe in the feme ratio ; and it 



N 



•will confequently be in general JV 0( '"''i and -^^ or C (V —L. ; that is, by compounding CJV (X _2_ 



>= /= 1 i or abfolute brightnefs a given quantity. M. Bouguer has carefully diftinguifhed intrinfic and 

 abfolute brightnefs, when he fpeaks of the quantity of light reflefted from a wall, at diiferent diftances; 

 Tpaite ttOpi^ue, page 39 and 40. 



Thi» 



