Prof. Draper on the Production of Light by Heat. 



397 



tain distance at which its light completely obliterates all traces of 

 the shadow. This distance is readily found ; for the disappear- 

 ance of the shadow can be determined with considerable exact- 

 ness. 



When 

 distances were as 



_ ^ — 7 _, 



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 presence of another sixty-four times as intense. 

 The precise number differs according to the sensibility of differ- 

 ent eyes, but for the same organ it is constant. 



Upon a paper screen I threw the shadow of a piece of copper, 

 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 shadow 

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

 lamp nearer before it was effaced. When this took place the dis- 

 tance 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. The in- 

 tensity of the light of the platinum is of course inversely propor- 

 tional to the square of the distance of the Argand lamp at the 

 foment of the obliteration of the shadow. 



Table of the Intensity of Light, emitted by Platinum at different Tem- 

 peratures. 



Temperature of 

 th e plati num. 



1900 

 2015 



2130 



2245 



2:360 

 24?^ 



2590 



Distance of Argand lamp. 



Experiment 1. 



Experiment 2. 



* • 



54 00 



54 00 



39-00 



4100 



2400 



24 00 



1800 



19 00 



14-50 



15-50 



1 1 50 



12-00 



900 



900 



Mean. 



Intensify of light 



• ■ • • i 



000 



54-00 



34 



4000 



0-62 



24-00 



1*73 



18-50 



2-92 



1500 



4-40 



11-75 



724 



900 



12-34 



In this table the first column gives the temperatures under ex- 

 amination in Fahrenheit degrees ; the second and third the dis- 

 tances of the Argand lamp from the screen, in English inches, in 

 two different sets of experiments : the fourth the mean of the 

 two: and the fifth the corresponding intensity of the ll '- nt - 



From this it is at once perceived, that the increase in the in- 

 tensity of the light, though slow at first, becomes very rapid as 



