434: W. LeConte Stevens — Experimental 



and their radiation while thus glowing was measured by means 

 of the thermopile. Finding his formula apparently satisfac- 

 tory, he assumed it to be applicable to yet higher temperatures. 

 Exposing his thermopile to the sun and comparing the deflec- 

 tion thus obtained with that given by a body of known temper- 

 ature and area, at a known distance, the temperature of the 

 sun was computed to be about 10,000° C. The work done by 

 Rosetti was valuable ; but his formula, involving two unde- 

 termined constants which vary with every thermometric instru- 

 ment employed, may be considered, like that of Dulong and 

 Petit, as of little more than historic interest. 



In 1888 an important communication regarding the radiation 

 of solid bodies was given to the Berlin Academy of Sciences 

 b} 7 Professor H. F. Weber, of Zurich.* He had been engaged 

 in an extended investigation of incandescent electric lamps for 

 the purpose of finding, if possible, a general formula for the 

 relation existing between the quantity of light emitted, the 

 area of the radiating surface, the quality of the substance 

 raised to incandescence, and the amount of electric energy 

 expended. Among the results attained was the establishment 

 of a formula expressing the relation between the intensity of 

 any selected homogeneous radiation, the corresponding wave 

 length, the temperature, and the quality of the radiating sub- 

 stance. An indispensable factor in the determination of this 

 formula was the previous admirable work of Langley on radia- 

 tion from the sun and various terrestrial sources, expressed in 

 an energy curve which is now familiar to all physicists. From 

 the study of this energy curve and of all other published 

 results that were accessible, as well as his own experiments, 

 Weber obtained the following formula : 



A 



Here T denotes the absolute temperature of the radiating 

 body, expressed in degrees centigrade, A the wave length of 

 the homogeneous radiation considered, F the area of the 

 radiating surface in square centimeters, e the base of the 

 ISTaperian system of logarithms, and s the amount of energy 

 radiated by the body in all directions in unit of time. The 

 constant, c, depends on the nature of the radiating surface; it 

 is the " emission constant." The constant F depends also on 

 the nature of the radiating body. Its mean value, as deter- 

 mined from a large number of experiments has been found 

 to be 



b" = 0-196 xl cr 6 



* Sitzungsberichte der K. Akademie der Wissenschaften, Berlin, 26 Juli, 1888. 



