1893.] President's Address. 391 



experimental investigation of " epipolic dispersion " (or " fluores- 

 cence"), and ; perhaps more than all, by his accurate measuring work, 

 from which he drew an exceedingly rigorous verification of the accuracy 

 of Huyghens' geometrical construction for the double refraction of 

 Iceland spar, Sir George Stokes has done much to make the Un- 

 dulatory Theory of Light sure and strong as it is a codification of 

 laws divined by Huyghens and Fresnel. But he has done more than 

 this. He has not merely left to mathematicians and speculative 

 physicists a desperate problem to find the dynamical explanation of 

 those laws. He has given (perhaps only in conversation) what 

 seems to me certainly the true clue to the dynamics of the Undula- 

 tory Theory of Light by pointing out that we must look not merely, 

 or not at all, to change of shape of the portion of ether within a 

 wave-length in the motion constituting light, but also, or altogether, 

 to its absolute rotation, for explaining the efficient force. 



ROYAL MEDAL. 

 Professor Arthur Schuster, F.R.8. 



Professor Schuster's first researches in physics were with the 

 spectroscope. Some of these date more than ten years ago. He has 

 been an important observer in more than one Solar Eclipse Expedi- 

 tion. The results he has obtained will be found in joint papers in 

 the 'Phil. Trans./ 1884 and 1889. He was associated with Lord 

 Rayleigh in one of his determinations of the ohm. In 1889 also he 

 showed that the diurnal magnetic changes could be accounted for by 

 a disturbing cause outside the earth's surface, and could not be 

 accounted for by a disturbing cause within the earth's surface. 



But perhaps the researches of most general interest are those on 

 electric discharge through gases. These will be found in the " Pro- 

 ceedings of the Royal Society," vols 42 and 47. In the former 

 paper he shows, amongst other things, that a steady current of 

 electricity can be obtained in air from electrodes at the ordinary 

 temperature, which are at a difference of potential of a quarter of a 

 volt only, provided that an independent current is maintained in the 

 same closed vessel. A vessel from which the air could be exhausted 

 was nearly divided into two parts by a metallic partition connected 

 to earth. A pair of electrodes was introduced on one side of the 

 partition between which a current could be passed from a battery of 

 many cells. A second pair was introduced into the vessel on the 

 other side of the partition, and these were connected to the poles of a 

 battery of low electromotive force. When high electric pressure was 

 applied to produce a current between the first pair of electrodes, a 

 current due to the battery of low electromotive force also passed 

 between the second pair. Professor Schuster supposes that the effect 



VOL. LIV. 2 E 



