MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 39 



were determined was that originally described by Bou- 

 guer, and subsequently used by Masson. After many 

 experiments, I found that it is the most accurate method 

 known. 



Any one who will endeavor to determine the intensi- 

 ties of light by Rumford's method of contrasting shad- 

 ows, or by that of equally illuminated surfaces, will find, 

 when every precaution has been used, that the results 

 of repeated experiments do not accord. There is, more- 

 over, the great defect that when the lights differ in color, 

 it is impossible to obtain reliable results except by re- 

 sorting to such contrivances as that described in the 

 Philosophical Magazine, August, 1844. 



Bouguer's principle is far more exact ; and where the 

 lights differ in color, that difference actually tends to 

 make the result more correct. As it is not generally 

 known, I will indicate the nature of it briefly : 



Let there be placed at a certain distance from a sheet 

 of white paper a candle, so arranged as to throw the 

 shadow of an opaque body, such as a rod of metal, on 

 the sheet. If a second candle be placed also in front of 

 the paper and nearer than the former, there is a certain 

 distance at which its light completely obliterates all 

 traces of the shadow. This distance is readily found, 

 for the disappearance of the shadow can be determined 

 with considerable exactness. When the lights are equal, 

 Bouguer ascertained that the relative distances were as 

 1 : 8, and therefore inferred correctly that in the case of 

 his eye the effect of a given light was imperceptible 

 when it was in presence of another sixty-four times as 

 intense. The precise number differs according to the 

 sensibility of different eyes, but for the same organ it is 

 constant. 



Upon a paper screen I threw the shadow of a rod of 

 copper, which intercepted the rays of the incandescent 



