1 08 J)r. J. 0. Bose. On the Determination of the 



and the deduced value of ^ would therefore be higher for slow oscil- 

 lations, the longer waves being thus the more refrangible. The 

 order of refrangibilities would in such a case appear to be some- 

 what analogous to that in an anomalously dispersive medium like 

 iodine vapour. 



With exceedingly quick ethereal vibrations which give rise to 

 light, there is an inversion of the above state of things, i.e., the 

 shorter waves are generally found to be the more refrangible. It 

 would thus appear that there is a neutral vibration region for each 

 substance at which this inversion takes place, and where a trans- 

 parent medium produces no dispersion. 



It would be interesting to be able to determine the indices of 

 refraction corresponding to different wave lengths, chosen as widely 

 apart as possible, and plot a curve of refrangibilities. A curve 

 could thus be obtained for rock salt, which is very transparent to 

 luminous and obscure radiations, and fairly so to electric radiation. 

 Carbon bisulphide, which is very transparent to all but the ultra- 

 violet radiation, would also be a good substance for experiment. 



For the construction of a curve of refrangibility for electric rays, 

 having different vibration frequencies, the indices could be deter- 

 mined by the method of total -reflection referred to above. The 

 determination of the corresponding wave-lengths, however, offers 

 great difficulties. Hertz used for this purpose the method of inter- 

 ference, the positions of nodes and loops of stationary undulation 

 produced by perpendicular reflection being determined by means of 

 tuned circular resonators. 



Sarasin and De la Rive subsequently repeated these experiments 

 with different sized vibrators and resonators. They found that 

 the apparent wave-length depended solely on the size of the 

 resonators. The wave-length found was approximately equal to 

 eight times the diameter of the circular resonator. From these 

 experiments it was supposed that the radiator emitted a continuous 

 spectrum consisting of waves of different lengths, and that the 

 different receivers simply resonated to vibrations with which they 

 happened to be in tune. If this supposition be true the emitted 

 radiation should, by the action of a prism, or better still, a^diffrac- 

 tion grating, spread out in the form of a continuous spectrum. If, 

 on the contrary, the radiation is monochromatic, the spectrum should 

 be linear. The experiments to be described below may throw some 

 light on this question. 



Professor J. J. Thomson, referring to the above case, is of opinion 

 that the hypothesis of a continuous spectrum is highly improbable. 

 It is more likely that, owing to the oscillation being of a dead-beat 

 character, the resonator is set in vibration by the impact of incident 

 electric waves. Each resonator vibrating at its particular free period, 



