penetrating into Space by Telescopes. 51 



"which may be extremely various, with regard to their power of 

 throwing out light more or less copiously. 



Brightness, I ascribe to bodies that throw out light ; and those 

 that throw out most are the brightest. 



It will now be necessary to establish certain expressions for 

 brightness in different circumstances. 



In the first place, let us suppose a luminous surface throwing 

 out light, and let the whole quantity of light thrown out by it 

 be called L. 



Now, since every part of this surface throws out light, let us 

 suppose it divided into a number of luminous physical points, 

 denoted by N. 



If the copiousness of the emission of light from every phy- 

 sical point of the luminous surface were equal, it might in 

 general be denoted by c ; but, as that is most probably never 

 the case, I make C stand for the mean copiousness of light 

 thrown out from all the physical points of a luminous object. 

 This may be found in the following manner. Let c express the 

 copiousness of emitting light, of any number of physical points 

 that agree in this respect ; and let the number of these points be 

 n. Let the copiousness of emission of another number of points 

 be c', and their number n'. And if, in the same manner, other 

 degrees of copiousness be called c 1 , c\ &c. and their numbers be 

 denoted by « a , n\ &c< then will the sum of every set of points, 

 multiplied by their respective copiousness of emitting light, give 

 lis the quantity of light thrown out by the whole luminous body. 

 That is, L = cn ^-c'n' -{- c z ?i x , &c. ; and the mean copiousness 

 of emitting light, of each physical point, will be expressed by 



cn-^-c n' +c* n z , G? c. 



N- ~ = C- 



Hi 



