PRELIMINARY NOTES ON MR. HALL'S RECENT DISCOVERY 199 







is well known that the rotation of the plane of polarization is opposite 

 in the two media, and Mr. Hall now finds his effect to be opposite in 

 the two media. This result I anticipated from this theory of the 

 magnetic rotation of light. 



But the formula makes the rotation inversely proportional to the 

 wave-length, whereas we find it more nearly as the square or cube. 

 This I consider to be a defect due to the imperfect theory ; and it would 

 possibly disappear from the complete dynamical theory. But the for- 

 mula at least makes the rotation increase as the wave-length decreases, 

 which is according to experiment. Should an exact formula be finally 

 obtained, it seems to me that it would constitute a very important link 

 in the proof of Maxwell's theory of light, and, together with a very 

 exact measure of the ratio of the electromagnetic to the electrostatic 

 units of electricity which we made here last year, will raise the theory 

 almost to a demonstrated fact. The determination of the ratio will 

 be published shortly; but I may say here that the final result will not 

 vary much, when all the corrections have been applied, from 299,700,000 

 metres per second; and this is almost exactly the velocity of light. We 

 cannot but lament that the great author of this modern theory of light 

 is not now here to work up this new confirmation of his theory, and 

 that it is left for so much weaker hands. 



But before we can say definitely that this action explains the rota- 

 tion of the plane of polarization of light, the action must be extended 

 to dielectrics, and it must be proved that the lines of electrostatic 

 action are rotated around the lines of force as well as the electric cur- 

 rents. Mr. Hall is about to try an experiment of this nature. 



I am now writing the full mathematical theory of the new action, and 

 hope to there consider the full consequences of the new discovery. 



Addition. I have now worked out the complete theory of the rota- 

 tion of the plane of polarization of light, on the assumption that the 

 displacement currents are rotated as well as the conducted currents. 

 The result is very satisfactory, and makes the rotation proportional to 



~ , which agrees very perfectly with observation. The amount of rota- 

 tion calculated for gold is also very nearly what is found in some of 

 the substances which rotate the light the least. Hence it seems to me 

 that we have very strong ground for supposing the two phenomena to 

 be the same. 



