Intelligence and Miscellaneous Articles. 563 



ON THE ABSOLUTE STANDARD OF LIGHT. BY PROF. J. VIOLLE. 



The investigation of the radiation emitted by melted silver had 

 essentially for its object the verification of the principle of that 

 method which consists in taking a metal at its fusing-point as 

 standard of light, a suggestion which I made at the International 

 Congress of Electricians in Paris in 1881. 



Preliminary investigations having established the constancy of 

 the radiation during the solidification, I was able to attempt with 

 certainty the realization of the absolute standard. 



I take as absolute unit of light the radiation from the surface of 

 1 square centim. of platinum at its solidifying-point. 



In order to get a definite multiple of the unit it is sufficient to 

 increase the surface. 



The Carcel lamp of Dumas and Kegnault's pattern having been 

 adopted as the usual secondary standard, I endeavoured in the first 

 place to settle its value compared with my principal standard. By 

 various methods, which closely agreed, I found for the value of the 

 standard Carcel lamp C, 



C- X 



from which, taking the surface into account, it is concluded that 

 the intrinsic intensity of our standard is almost eleven times that of 

 the Carcel lamp. 



I also worked with electrical foci. The comparison with our stan- 

 dard was found to be satisfactory in practice. I shall merely adduce 

 experiments with incandescent lamps, the constancy and colour of 

 which render the comparison with platinum easy and certain. 



A Swan lamp was fed by a battery of thirty Kabath accumu* 

 lators. By means of a resistance-box in the circuit, the conditions 

 could be varied. An observer noted each minute the strength of the 

 current i, and the fall of the potential between the terminals of 

 the lamp. It was only necessary therefore to take the photometric 

 measurement, and note the corresponding condition of the lamp, to 

 get its illuminating-power E, special experiments made at the 

 beginning and end of each series having established this illumina- 

 ting-power for different values of ie. The relative values of E given 

 below are independent of possible variations of CarceFs lamp. 



In order to compare the electric lamp with the standard, a 

 Bunsen's photometer was used which could be moved about between 

 the two sources, which were at a distance from each other of 4 metres. 

 The rays emitted by the platinum being directed horizontally by 

 means of a mirror of 45°, the total effective distance was about 

 4*54 metres. It was each time determined exactly ; we shall 

 denote it by A, and shall call D the distance of the platinum from 

 the photometer. 



In the first set of experiments three different observers made 

 each six measurements. Of the eighteen determinations thus 



