478 ROSA and vinal: silver voltameter 



what different interpretation. Imposing the stigmatic condition 



has eliminated both n and d with their possible variations. It 



is well known from experimental data that this form of equation 



holds well and that the stigmatic condition is frequently fulfilled. 



For gray radiation \og(E/J) is by definition independent 



of wave length. In case C\, n and C 2 are all constant, as with 



equilibrium radiation, the stigmatic condition can hold for but 



a limited range of wave lengths. The gray condition in the 



general case with varying parameters gives 



, d \-" +5 C 2 - B 2 



log = constant + — — — 



jDj A/ 



This is consistent with the stigmatic condition (7) for free radia- 

 tion, but either may be true without the other. 



We have shown that the logarithmic isochromatic lines repre- 

 senting equilibrium radiation do not form a stigmatic pencil 

 except for a limited range of wave lengths. For free radiation, 

 the stigmatic condition gives an equation known to be of wide 

 validity. The stigmatic condition and the condition for gray- 

 ness may both be satisfied, but either may hold without the other. 



PHYSICS. — Summary of experiments on the silver voltameter at 

 the Bureau of Standards. 1 E. B. Rosa and G. W. Vinal, 

 Bureau of Standards. 



The investigation of the silver voltameter at this Bureau was 

 first begun by the late Dr. K. E. Guthe in 1904. His results 

 were published in two papers about a year later. In 1907 the 

 work was again taken up by Dr. N. E. Dorsey in cooperation 

 with the present authors, but the results obtained at this time 

 did not confirm the newly published experiments of the National 

 Physical Laboratory, and new difficulties arose which were not 

 understood. These experiments were not .published. In the 

 following year the work was resumed and preparations were 

 made for a very thorough study of the silver voltameter. The 

 voltameter received added importance when the ampere was 

 adopted by the London Electrical Congress as the second funda- 



1 A more detailed summary will appear as Bureau of Standards Scientific 

 Paper No. 285 (Bull. Bur. Stds., 13: 479-514. 1916). 



