S54' Prof. Draper on the product ion of Light by Heat. 



result more perfect. As it is not generally known, I will in- 

 dicate the nature of it briefly. 



Let there be placed at a certain distance from a screen of 

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

 a ruler, or other opake body, on the screen. If a second can- 

 dle 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 found that the relative distances were as 1 : 8 ; he 

 inferred therefore, correctly, that in the case of his eye, the 

 effect of a given light was imperceptible when it was in pre- 

 sence 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 piece of cop- 

 per, which intercepted the rays of the incandescent platinum : 

 then taking an Argand lamp, surrounded by a cylindrical 

 metal shade through an aperture in which the light passed, 

 and the flame of which I had found by previous trial would 

 continue for an hour almost of the same intensity, I approached 

 it to the paper until the shadow cast by the copper disappeared. 

 The distance at which this took place was then measured, and 

 the temperature of the platinum determined. 



The temperature of the platinum was now raised ; the sha- 

 dow became more intense, and it was necessary to bring the 

 Argand lamp nearer before it was effaced. When this took 

 place the distance of the lamp was again measured, and the 

 temperature of the platinum again determined. 



In this manner I obtained several series of results, one of 

 which is given in the following table. They exhibited a more 

 perfect accordance among each other than I had anticipated. 



Table of the Intensity of Light emitted by Platinum at differ- 

 ent Temperatures. 



