52 Dr. Herschel on the Power of 



It is evident that the mean power, or copiousness of throwing 

 out light, of every physical point in the luminous surface, mul- 

 tiplied by the number of points, must give us the whole power 

 of throwing out light, of the luminous body. That is CN—L. 



I ought now to answer an objection that may be made to this 

 theory. Light, as has been stated, is transparent ; and, since 

 the light of a point behind the surface of a flame will pass 

 through the surface, ought we not to take in its depth,, as well 

 as its superficial dimensions ? In answer to this, I recur to what 

 has been said with regard to the different powers of throwing 

 out light, of the points of a luminous surface. For, as light must 

 be finally emitted through the surface, it is but referring all 

 light arising from the emission of points behind the surface, 

 to the surface itself, and the account of emitted light will be 

 equally true. And this will also explain why it has been stated 

 as probable, that different parts of the same luminous surface 

 may throw out different quantities of light. 



Since, therefore, the quantity of light thrown out by any lu- 

 minous body is truly represented by CN, and that an object is 

 bright in consequence of light thrown out, we may say that 

 brightness is truly defined by CN. If however, there should at 

 any time be occasion for distinction, the brightness arising 

 from the great value of C, may be called the intrinsic bright- 

 ness ; and that arising from the great value of N, the aggregate 

 brightness ; but the absolute brightness, in all cases, will still be 

 defined by CN. 



Hitherto we have only considered luminous objects, and their 

 condition with regard to throwing out light. We proceed now 

 to find an expression for their appearance at any assigned dis- 

 tance ; and here it will be proper to leave out of the account, 



