[ 175 ] 



XXIV. On certain Objections to the Theory of the Equivalent of 

 Refraction. By Dr. Albreciit Schrauf, Custodian of the 

 Imperial Miner alogical Cabinet of Vienna *. 



ON a cursory examination of the optical literature of the past, 

 we find numerous essays, of various dates, which, being 

 based upon the theory of emission, have denied the correctness 

 of the hypothesis of vibration. The idea of refractive power in- 

 troduced into optics by Newton appears to have the same pur- 

 port (Schicksal) . Besides these older investigations, the most 

 recent work of Riihlrnannt are intended to show the inapplica- 

 bility and incorrectness of the refractive power. Since in the 

 above-named investigation several of my previously published 

 conclusions on this subject are declared to be erroneous, I take 

 this opportunity of placing these objections in their proper light. 



I imagine, moreover, that there are many assertions in M. 

 Ruhlmann' s criticism which he would not have published if he 

 had not ignored, for reasons unknown to me, my later investiga- 

 tions — both those on " Equivalents of Refraction" J (1865), and 

 my "-Studies "§ published in December 1866. These two in- 

 vestigations, compared with my former publications (1862), con- 

 tain many new points of view, and precisely in that very field 

 which M. Ruhlmann has chosen as the starting-point of the 

 discussion. It appears, indeed, that the doubts now published 

 by M. Ruhlmann concerning the theory of refractive power were 

 considered and explained by me in the course of the preceding- 

 year. 



If we examine the results of M. Ruhlmann' s important work 

 on the Thermo-optical Constants of Water, the first point which 

 demands some attention is the sentence on page 202 1|, "That 

 the magnitude M given by Schrauf is by no means a constant, 

 but continually varies with the temperature." 



The variation of the refractive power with the temperature, 

 however, is not unknown to me, and received my mature 

 consideration long ago. In my "Studies," p. 69, I stated 

 a year previously, "A uniform result of the calculation, 

 however, is that the values of M (refractive power) diminish 

 with increasing temperature. The obvious objection here 

 arises, that, in consequence of this observation, the function 



* Translated by Frederick Guthrie, F.R.S.E., from a separate impres- 

 sion, communicated by the author, from PoggendorfF' sAnnalen, vol. exxxiii. 

 p. 479. 



f PoggendorfFs Annalen, vol. exxxii. p. 193. 



X Schrauf, " Die Refractionsaequivalente und optisehen Atomzahlen der 

 Grundstoffe," Sitzungsb. der Wien. Akod. 1865, vol. lii. (2) p. 1/6. 



§ Schrauf, Physikalische Studien uber die gesetzm'dssigen Beziehungen 

 von Materie und Licht. Wien, Gerold, 1867, p. 249. 



|! Pogg.i4wM.vol. exxxii. Phil. Mag. S. 4. vol. xxxv. p. 395. 



