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XLIL Attempt at a Theory of the (Anomalous) Dispersion of 

 Light in Singly and Doubly Refracting Media. By Professor 

 E. Ketteler*. 



AFTER establishing, as I believe, by my own experiments, 

 especially on gases, as well as making critical use of all 

 extant observations, the empirical side of the theory of disper- 

 sion as a whole, I have further been able also to explain its 

 connexion with the elliptical polarization of reflection and 

 refraction f- The dispersion-formula constructed by me in- 

 cludes in itself the very phenomenon, afterwards discovered, 

 of anomalous dispersion ; and it satisfies experience not only 

 for the more feebly absorbed transmitted rays, but also, appa- 

 rently, for the perfectly dark regions' of the spectrum, which 

 are accessible only by the elliptical polarization of the reflected 



light. 



Anomalous dispersion has meanwhile effected a complete 

 revolution in reference to theory ; for it has become evident 

 that in dioptric processes the aether alone will not suffice ; and 

 so the view represented chiefly by Boussinesq, Sellmeier, and 

 myself, that the gethereal and corporeal particles vibrate 

 together, is gradually more and more making way. And then 

 a really tenable theory of dispersion, based on this conception, 

 promises, together with a more profound understanding of 

 light-motion in general, some disclosures concerning the struc- 

 ture and constitution of the aggregates, composed of sethereal 

 and corporeal matter, which form the only ponderable media. 



The most recent treatise on anomalous dispersion we owe to 

 HelmholtzJ. He also starts from a convibration of ponderable 

 particles ; and by assuming only the simplest possible mecha- 

 nism in reference to the reciprocal action between them and 

 the aether particles, he readily arrives at the construction of the 

 two requisite differential equations. Helmholtz rejects the 

 assumption, made by the older physicists, that the sethereal 

 and corporeal particles attract or repel each other point for 

 point, and substitutes for it an action like that of a vibrating 

 pendulum on the surrounding air. Lastly, he shows emphati- 

 cally that, on account of the gradual conversion of light into 

 heat (that is, absorption of the regular oscillatory motion), co- 



* Translated from a separate impression, communicated by the Author, 

 from the Verliandlungen des naturhistorischen Vereins der preussisclien 

 Rheinlande und Westphalens, Jahrg. xxxiii., 4th series, vol. ii. 



t "Das Complexe als Ausdruck des Zusanimenhangs zwischen der 

 elliptischen Polarisation der Spiegelung imd Brechung und der Dispersion 

 der Farben," Verh. des naturhist. Vereins jur Rheinland-Westphalen, 1875. 



\ Monatsber. der Berl. Akad, Oct. 1874 ; Pogg. Ann. vol. cliv. p. 582. 



