530 M. Helmlioltz on the Theory of Electrodynamics. 



so much feebler than the recurrent image that it produces no 

 alteration in its tint. 



In conclusion I would remark upon the apparent analogy 

 between the phenomenon of recurrent vision and that of in- 

 duced currents in electricity. A nerve-current in one kind of 

 nerves appears to induce nerve-currents in the other kinds in a 

 manner analogous to that by which a current of electricity in one 

 conducting wire induces currents in parallel conducting wires. 



Leeds Grammar School. 

 November 9, 18/2. 



LXIII. On the Theo?'y of Electrodynamics. -Bz/M.Helmhgltz*. 

 IHE theory of electrodynamic actions, besides its immediate 



T' 



value for the understanding of this important and proUfic 

 branch of physics, is more universally interesting in its relation 

 to the fundamental principles of general mechanics. All the 

 other known actions at a distance can be easily and completely 

 reduced to attractive and repulsive forces of points of masses, 

 while the intensity of these forces depends only on the reciprocal 

 distances of the points and not on their motion. Moreover the 

 hitherto known actions between molecules can either be entirely 

 referred to such forces, or at least are so similar in their v/hole 

 manner of appearance to the effects produced by gravity that we 

 find no difficulty in imagining them the effects of forces similar 

 in character. But the electrodynamic forces constitute an ex- 

 ception. They form a class of distant actions produced only by 

 the state of motion of the efficient agent, the electricity, — a state 

 of motion which makes itself perceptible as such by a whole 

 series of phenomena — by development of heat in solid conduc- 

 tors, chemical decomposition in liquid conductors, &c. The real 

 laws of the manner of appearance of these forces are, in the main^ 

 well known, and have been reduced by F. E. Neumann, Sen., to 

 a comparatively simple expression, which, however, gives not 

 the action of mass-point upon mass-point, but of one linear 

 element of a current upon the other. I have myself given to 

 Neumann's expression of the potential a more general formf^ in 

 which it embraces also the diff'ering expressions resulting from 

 the theories of W. Weber and Maxwell for the potential of each 

 two current-elements. For closed currents all these expressions 

 give the same results ; on the contrary, for open ones, the actions 

 of whi-ch have, indeed, at present been little investigated, they 



* Translated from the Monatsbericht der Kon. Preuss. Akad. d. Wis- 

 senschaften zu Berlin for April 1872. 

 t Journal fiir reine und angewandte Mathematik, vol. Ixxii. 



