Penetration into Space by Tdefcepes. 49^ 



nous atmofphere iu different places*. This regards only bodies that are felf-lurhinous. 

 But the greateft inequalities in the brightnefs of luminous bodies in general, will undoubt- 

 edly be owing to their natural texture ; which may be extremely various, with regard to 

 their power of throwing out light more or lefs copioufly. 



Brightnefs, I afcribe to bodies that throw out 'light; and thofe that throw out moft are 

 the brighteft. 



It will now be neceflary to eftablifh certain expreffions for brightnefs in different cir- 

 cumflances. 



In the firft place, let us fuppofe a luminous furface throwing out light, and let the whole 

 quantity of light thrown out by it be called L. 



Now, (ince every part of this furface throws out light, let us fuppofe it divided into a 

 number of luminous phyfical points, denoted by A/^. 



If the copioufnefs of the emiffion of light from every phyfical point of the luminous fur- 

 face were equal, it might in general be denoted by c ; but, as that is moft probably never 

 the cafe, I make C ftand for the mean copioufnefs of light thrown out from all the phyfical 

 points of a luminous objedl. This may be found in the following manner : let c exprefs 

 the copioufnefs of emitting light, of any number of phyfical points that agree in this rc- 

 fpeft ; and let the number of thefe points be «. Let the copioufnefsof emiffion of another 

 number of points be c, and their number ti . And if, in the fame manner, other degrees 

 of copioufnefs be called c% c^, bfc. and their numbers be denoted by «% «3, ^c. then will 

 the fum of every fet of points, multiplied by their rcfpeftive copioufnefs of emitting light, 

 give us the quantity of light thrown out by the whole luminous body. That Is, L = en 

 + c n + £««% i^c. ; and the mean copioufnefs of emitting light, of each phyfical point, 



en -{■ c ti 4- c^n", l^c. 

 will be expreffed by -^ — C. 



It is evident that the mean power, or copioufnefs of throwing out light, of every phyfical 

 point in the luminous furface, multiplied by the number of points, muft give us the whole 

 power of throwing out light, of the luminous body. That is C N :zz L. 



I ought now to anfwer an obje£lion that may be made to this theory. Light, as has 

 been ftated, is tranfparent ; and, fince the light of a point behind the furface of a flame will 

 pafs through the furface, ought we not to take in its depth, as well as its fuperficial di- 

 menfions ? In anfwer to this, I recur to what has been faid with regard to the differeni: 

 powers of throwing out light, of the points of a luminous furface. For, as light muft be 

 finally emitted through the furface, it is but referring all light arlfing from the emiffion of 

 points behind the furface, to the furface itfelf, and the account of emitted light will be 

 equally true. And this will alfo explain \irhy it has been ftated as probable, that different 

 parts of the fame luminous furface may throw out different quantities of light. 



• See the Paper on the Nature and Confti-uilion of the Sun. Phil. Tranf. for J795, page 46, 



Vol. IV, — February 1801. 3S Since 



