96 ON THE KELATION OF OPTICS TO PAINTING. 



mixtures ; while the darkest parts are produced with 

 the same black. Both, being hung on the same wall, 

 share the same light, and the brightest as well as the 

 darkest parts of the two scarcely differ as concerns 

 the degree of their brightness. 



How is it, however, with the actual degrees of 

 brightness represented? The relation between the 

 brightness of the sun's light, and that of the moon, 

 was measured by Wollaston, who compared their in- 

 tensities with that of the light of candles of the same 

 material. He thus found that the luminosity of the 

 sun is 800,000 times that of the brightest light of a 

 full moon. 



An opaque body, which is lighted from any source 

 whatever, can, even in the most favourable case, only 

 emit as much light as falls upon it. Yet, from Lam- 

 bert's observations, even the whitest bodies only reflect 

 about two fifths of the incident light. The sun's rays, 

 which proceed parallel from the sun, whose diameter 

 is 85,000 miles, when they reach us, are distributed 

 uniformly over a sphere 195 millions of miles in dia- 

 meter. Its density and illuminating power is here 

 only the one forty-thousandth of that with which it 

 left the sun's surface ; and Lambert's number leads to 

 the conclusion that even the brightest white surface 

 on which the sun's rays fall vertically, has only the 

 one hundred- thousandth part of the brightness of the 



