32 Prof. G. F. FitzGerald. 



called the syntony of the matter and light vibrations, and is conse- 

 quently dependent on the frequency of the matter vibrations. In 

 most substances the dispersion is controlled within the visible 

 spectrum by a great absorption band in the ultra-violet, this band 

 representing a possible frequency of vibration of the molecules, i.e., of 

 that part of the molecules which affects the ether, be it electrons or 

 something which simulates the actions ascribed to electrons. If 

 owing to any cause this absorption band be changed in position, i.e.,. 

 the frequency of the molecular vibrations be altered, the dispersion 

 of the medium will be changed, and with it the velocity of propaga- 

 tion of light within the visible spectrum. Now Zeeman has shown 

 that in a magnetic field the frequency of vibration of molecules 

 producing light circularly polarised in one direction is different from 

 that of molecules producing light circularly polarised in the opposite 

 direction, and that consequently the absorption bands for molecules 

 in a magnetic field for oppositely circularly polarised waves will 

 differ. Hence we conclude that the velocity of propagation of 

 oppositely polarised waves within the visible spectrum will differ, 

 and that is the Faraday effect. Hence these two phenomena are 

 directly connected with one another, independently of any other than 

 general dispersion theory, i.e., independently of any theory of 

 electrons, such as Lorentz has shown will explain the Zeeman 

 effect. If we introduce such a theory, and use it to explain the 

 Faraday effect upon the lines now laid down, we arrive at the inter- 

 esting conclusion that before the Zeeman effect was observed the 

 Faraday effect would have shown that in the majority of substances 

 we must assume the ether vibrations to be due to the motion of a 

 negative electron. In substances with a negative Faraday effect, 

 such as some magnetic bodies, we may conclude either (a) the ether 

 vibrations are due to a positive electron, or (fr) the absorption band 

 controlling the dispersion is in the ultra-red, or (c) the cause of the 

 Faraday effect may be due rather to a difference of intensity of the 

 absorption band for oppositely circularly polarised vibrations than 

 to a difference of frequency. This latter alternative is in some ways 

 the most consonant with the usual theory as to the difference 

 between paramagnetic and diamagnetic bodies. 



If we proceed to calculate what amount of Faraday effect might 

 be expected from the observed value of the Zeeman effect, we are 

 met by the difficulty of obtaining data. "We require to know, for 

 some one or more substances, the amount of the Zeeman effect, the 

 amount of the Faraday effect, and the ordinary dispersion of the 

 substance for light. The only gas (and it is only in gases that the 

 Zeeman effect has been observed) for which the dispersion and 

 Faraday effect are well ascertained, is air, and for it the amount of 

 the Zeeman effect has not been published. In preparation, however, 



