AT THE PHYSICO-TECHNICAL INSTITUTE 407 



phenomenon appear to be indubitably correct, but there are 

 some things which I do not altogether understand, e. g. that the 

 reflection of the green leaves of a plant are only seen in one 

 given direction when the plate is rotated in its plane. . . . 



' I had hoped to present myself in person to my new colleagues 

 at the Academy this August, but was deterred by the fear of 

 cholera quarantine/ 



On December 15, 1892, Helmholtz gave a paper to the Berlin 

 Academy which he called 'The Electromagnetic Theory of 

 Colour Dispersion ', at which he had been working for a long 

 time. 



The prevailing theories of Optics had rejected the notion 

 that light waves could be other than elastic in their nature. 

 Maxwell, in the treatise published in 1865 with the title Electro- 

 magnetic Theory of Light, had linked together two conjectures, 

 originally far apart, in such a way that they gave each other 

 mutual support. Electricity in motion produces magnetic 

 forces, magnetism in motion produces electrical forces, but 

 these effects are only perceptible with very great velocities. 

 The constant which controls the reciprocal relations between 

 electricity and magnetism is a very high velocity, which proves 

 equal to the velocity of light. The explanation of colour dis- 

 persion on the ground of the electromagnetic theory of light is 

 only possible with regard to the ponderable masses which are 

 embedded in the ether, since the dispersion of light belongs to 

 those processes, which, like refraction, galvanic conductivity, 

 the accumulation of true electricity, and the existence of 

 magnetic poles, never take place in the pure ether of a vacuum, 

 but only in, or at, the border of spaces which contain ponderable 

 masses as well as the ether. It was recognized by Helmholtz 

 that, according to Maxwell's mathematical theory, pondero- 

 motive forces must be active within ether that was permeated 

 by electrical oscillations, and might set the heavy atoms that lie 

 within the ether in motion. But if the ponderable particles are 

 not themselves electrical, these forces must be proportional to 

 the squares of the electrical and magnetic momentum of the 

 oscillating ether, and therefore would have the same magnitude 

 and direction for negative as for positive values. During each 

 vibration period, accordingly, they would twice reach their 

 maximal and twice their minimal value, so that they could not 



