542 Theoretical Optics since 1840. 



On such an hypothesis the molecular velocity of the 

 aether might measure magnetic force, while electric dis- 

 placement would then be proportional to the curl of the 

 twist, or we might adopt the analogy suggested by 

 Heaviside (' Electrician,' Jan. 23, 1891), and developed, as 

 I have said, by Larmor, according to which the kinetic 

 energy measures the magnetic force and the twist the electric 

 displacement. 



The electromagnetic theory, though it does not rest on a 

 mechanical basis, has linked together optical and other phe- 

 nomena in a striking fashion. The advance from the days of 

 Green has been a great one. 



And leaving now the general theory, the development of 

 its details has not been less striking. On all sides there 

 has been advance, and along most of the lines of advance 

 Stokes was a pioneer. 



Newton's difficulty in accepting the undulatory theory was 

 really solved when Young enunciated the principle of inter- 

 ference, but it needed Fresners experiments to convince men 

 of its truth. It was clear, of course, that the effect at any 

 point due to a wave of light could be calculated by finding 

 the effect due to each element of the wave and summing 

 these; but Stokes, in his papers on diffraction (1851), was the 

 first to establish a correct expression for the effect produced at 

 a distant point by an element of the wave and to show how 

 these effects were to be summed. 



The germ of all that has been discovered by means of 

 spectrum analysis is contained in his explanation of Kirchhoff's 

 original experiment, often quoted by Lord Kelvin, and from 

 his paper on "Fluorescence" have sprung the modern theories 

 of dispersion, including anomalous dispersion. On this point 

 the note he has added to this paper in the third volume of his 

 Collected Works has a special interest. Although he did not 

 fathom the connexion between aether and matter, and, on the 

 whole, the criticisms passed by later writers on his theory of 

 aberration are to be accepted as justified, his papers must be 

 studied by any one who is anxious to penetrate the mystery, 

 and did much to put the facts in a clear light. 



My survey is, I realize, entirely inadequate ; it is but a frac- 

 tion even of the corner of the field I set out to examine that I 

 have covered, but I must stop. I have said enough, I hope, to 

 show that progress has been continuous and marked, and 

 in no small degree that progress has been due to the work 

 of Sir George Stokes. 



I had intended to bring before you some more practical 

 questions connected with the work of my own Laboratory. 



