AT THE PHYSICO-TECHNICAL INSTITUTE 409 



presents a correct account of anomalous dispersion. Helm- 

 holtz shows that in the equations of motion to be established, 

 electrical momentum must (since there is as yet no electrical 

 force, no inertia, no friction, &c.) be distinguished from 

 that of the free ether, and the undulatory vibrations must be 

 investigated separately in the free ether and in that charged 

 with mobile molecules, deducing from the mathematical ex- 

 pressions here developed, that the normal dispersion spectrum 

 can be produced by absorptions in the ultra-violet. An error in 

 the mathematical development which led to contradictions 

 between Helmholtz's theory and the earlier theories of dis- 

 persion, was detected by Reiff at a later time. 



A peculiarly interesting corollary is that phase-differences 

 exist between the electrical and the magnetic vibrations, as well 

 as between the electrical vibrations and those of the ions, so that 

 intense vibrations might conceivably tear the ions away from 

 their combinations, especially where there is also an electrostatic 

 charge of the substance. It would follow that the escape of 

 electricity under the influence of the violet rays, as observed by 

 Hertz, might obtain for all substances in which there is strong 

 absorption at the limits of the ultra-violet. For non-absorbent 

 media the theory leads to a dispersion formula which approxi- 

 mates to that of Cauchy ; complete polarization is produced by 

 refraction, and if the electrical vibrations are assumed to be in 

 the plane of incidence, Fresnel's value for the intensity of the 

 reflected ray is obtained in the other direction of polarization. 

 An appendix gives the verification of the dispersion formula 

 by Fraunhofer's experiments, which yielded very satisfactory 

 results. 



On the Jubilee for the fiftieth year of the doctorate of his 

 oldest friend, du Bois-Reymond, Helmholtz composed an 

 address, in February of the same year, at the request of the 

 Academy of Sciences, in which he gave expression to his own 

 deep affection, and to the high appreciation which he felt for the 

 work of his former colleague in physiology. 



After Helmholtz had brought the winter lectures to a close, 

 and had attended an interesting meeting of the Aeronautic 

 Commission (he was present at the third great ascent, which 

 took place in the early days of March, but was not entirely satis- 

 factory), he went in April to Ruhrort for the wedding of his niece, 



