494 PROCEEDINGS OF THE AMERICAN ACADEMY 



Investigations on Light and IIeat, made and pubushed wholly or in pabt with 

 Appeoprution fkom the Rumford Fund. 



XXYII. 



CONTRIBUTIONS FROM THE THYSICAL LABORATORY OF 

 HARVARD UNIVERSITY. 



A STANDARD OF LIGHT. 



By John Trowbridge. 



Communicated May 26, 1885. 



The discussions in the Paris Conference of 1881-84 upon the sul> 

 ject of a standard of light, which resulted in the adoption of the light 

 emitted by a surface of platinum at the point of solidification, seemed 

 to assort ill with the discussions wliich led to a reaffirmation of the 

 value of the C. G. S. system of absolute physical units, and a recogni- 

 tion of the relations between work and heat, and electrical energy. 



The solidification point of platinum may be a fixed point in nature ; 

 but it has not been shown how this fixed point can be connected with 

 that great web of physical measurements wl)icb has been woven by 

 Weber, Ilelmholtz, Thomson, Maxwell, and other physicists. It is true 

 that durinff the discussions of the Conference reference was made to a 

 proposition of Schwendler, that the light emitted by a strip of platiimm 

 rendered incandescent by a known electrical current should be taken 

 as a standard. This proposition, however, received little support ; 

 and the Conference finally adopted the light emitted by solidifying 

 platinum as a standard. 



It, seems highly desirable that any standard of light which may be 

 adopted should be connected with the present system of absolute 

 measurements. The suggestion of Schwendler, therefore, seems to 

 merit more attention than it has received. The suggestion of employ- 

 ing the light from a strip of platinum rendered incandescent by an 

 electrical current is really due to Dr. John W. Draper, of New York, 

 who in 1847 enunciated it as follows: "A surface of platinum of 

 standard dimensions raised to a standard temperature by a voltaic 

 current will always emit a constant light. A strip of that metal one 



