2 88 HERMANN VON HELMHOLTZ 



have electro-dynamic effects, required a knowledge not only 

 of the indeterminable constant k, but also of the dielectric 

 constant of the air, or the velocity of transverse electric waves 

 in air, which likewise could not be determined from previous 

 experiments. It was thus of primary importance to determine 

 this latter constant by experiments, which were accordingly 

 undertaken by Boltzmann in his laboratory, with the view of 

 testing Clerk Maxwell's now famous electromagnetic theory 

 of light. This distinguished physicist, whom Helmholtz vainly 

 endeavoured to secure for Berlin at a later time as successor to 

 Kirchhoff, wrote to Konigsberger in April, 1902, that in conse- 

 quence of Helmholtz's supposition that Maxwell held the 

 refractive indices to be equal to the dielectrical constants, the 

 requisite agreement was not obtained ; he therefore left Berlin 

 in the firm conviction that Maxwell was entirely wrong, and was 

 on the point of printing his criticism of the theory. As early as 

 November i, 1872, however, he wrote to Helmholtz : ' I cannot 

 forbear to tell you of another thing. I was always under the 

 impression (and I believe you expressed the same idea when I 

 was in Berlin) that on Maxwell's theory of the identity of light 

 and electricity, the dielectric constants which I had determined 

 must always be equal to the refractive indices. On now 

 putting the values of all the dielectric constants together in 

 a table, I was much worried at their deviating so far from the 

 refractive indices, but noticed at the same time that they were 

 always nearly equal to the squares of the latter. The thought 

 flashed through my mind that Maxwell's theory might require 

 this, since the velocities of transmission are always proportional 

 to the square root of the forces. I looked up Maxwell's 

 treatise, and there sure enough was plain to read that the 

 dielectric constants must be proportional to the squares of 

 the refractive indices (the magnetic induction constant is about 

 equal to unity for all these substances) ; so that I must look 

 on my experiments as a confirmation of Clerk Maxwell's 

 Theory.' 



Neither from theory nor experiment was it possible as yet 

 to decide for one or other of the hypotheses mentioned by 

 Helmholtz in his first paper, and both in the note laid before 

 the Academy on April 18, 1872, ' On the Theory of Electro- 

 dynamics/ and in the article that appeared in the //. fur reine 



