234 Mr. S. S. Richardson on Magnetic Rotary 



to look for confirmation of the value of \ x from other 

 sources. 



In a paper on " Light Absorption and Fluorescence "* 

 Prof. E. C. 0. Baly has shown that the dispersion of water- 

 vapour, hydrogen chloride, chlorine, ammonia, and nitrous 

 oxide in the visible spectrum can be accurately calculated 

 from a simplified Sellmeier formula by taking a dispersional 

 frequency which is some integral multiple of the frequency 

 of some one of the infra-red bands. In the case of water- 

 vapour for example, taking the infra-red period at 2*95 /-t, if 

 we put Ai = 2*95/31 the calculated dispersion curve is steeper 

 than that observed; with Ai = 2*95/33 it is less steep than 

 the observation curve; but with X 1 = 2*95/32 the agreement 

 is remarkably good, and it does not appear that a better 

 result would be obtained by taking an intermediate value. 

 Prof. Baly's argument rests upon the theory of energy 

 quanta and the supposition that the absorbed energy is 

 emitted at an infra-red frequency. A radiation theory of 

 absorption appears to be quantitatively inadequate in the- 

 case of solids, but it is of course possible that some other 

 mechanism may account for the interdependence of ultra- 

 violet and infra-red frequencies. In Part II. of the same 

 paper Prof Baly points out that the central frequencies of 

 the absorption systems in the near ultraviolet of the vapours 

 of benzene, toluene, and the xylenes are in each case almost 

 exact multiples of the frequencies of certain infra-red bands^ 

 If similar relations can be proved to exist in general for 

 solids and liquids, they will provide a valuable aid in the 

 development of optical theory. There are some indications 

 that this may be possible. The absorption band of carbon- 

 bisulphide at *321 fju corresponds to the infra-red band at 

 3*2 fi observed by Ooblentz, and Martens has shown that this 

 band (*321) gives rise to a small amount of selective disper- 

 sion. ( Cf. also the result for benzene, infra.) 



An alternative method of determining \ lf X 2 , . . . . is 

 provided by the Faraday effect, and since the application of 

 an intense magnetic field induces a slight variation in the 

 periods of the resonators, it is to be expected that the 

 phenomenon of magnetic rotary dispersion may furnish in- 

 formation not to be obtained from natural dispersion alone. 



The general effect of the application of a magnetic field 

 to a material medium is to superpose on the existing motions 

 of the electrons or electrified particles a uniform angular 

 velocity eH/2mC ; the effect of which, in turn, is mainly a 



* Phil. Mag. April 1914. 



