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LXYIIL Anomalous Dispersion or the Magnetic Rotation of 

 the Plane of Polarization. B>i l\. W« WOOD*. 



^l^HE question of the form of the curve representing the 

 A magnetic rotatory dispersion of absorbing media in 

 the vicinity of the absorption-band is one of considerable im- 

 portance. The formula for the rotation, developed from the 

 hypothesis of molecular currents, calls for anomalous dispersion, 

 i. e. a dispersion-curve with oppositely directed 1 tranches at 

 the edges of the band, while the equation built up on the 

 " Hall effect n hypothesis calls for rotations of similar sign on 

 site sides of the band (see Drude's 'Optics''). The latter 

 condition occurs in the case of sodium vapour as is well known, 

 while the loner series of investigations by Schmauss, published 

 in the Annalen dev Physik during the past three years, 

 appeared to establish the fact that curves of the former type 

 were exhibited by solutions of the aniline dyes and solutions 

 of the salts of didymium. Having found that ordinary 

 anomalous dispersion could be shown to much better advan- 

 o I >y means of prisms of fused cyanine than by means of 

 alcoholic solutions, I made an attempt to verify the results of 

 Schmauss by employing thick films of this substance formed 

 b y pressing the fused dye between plates of glass, which were 

 then separated by the blow of a hammer. The film was then 

 detached from the glass plate, to avoid the rotation produced 

 by the glas-. and mounted between the poles of a powerful 

 electromagnet. Absolutely no trace of magnetic rotation 

 could be detected, however, even when the films were -o 

 thick that only red light could be made to pass through them. 

 Solutions were also tried, but so far as could be determined 

 with the apparatus employed, the dye was without effect upon 

 the rotatory power of the solvent. At about the time of the 

 completion of these experiments, a paper by Bates appeared 

 in the Annalen (Ann. der Physik, xii. pp. 1080, 1091), who 

 pointed out that the method employed by Schmauss was open 

 to objections, and that his anomalous carves could be explained 

 as due to error- in the determination of the position of the 

 dark band in the spectrum. Bates repeated the work by a 

 method which was claimed to be many times more accurate 

 than that employed by Schmauss, and free from the objections 

 to which the latter was open. The results of his experiments 

 absolutely negative ; that is, the dye was found to be 

 without influence upon the magnetic rotatory power of the 

 liquid in which it was dissolved. The character of the work 



* Communicated by the Author, 



