176 Dr. A. Schrauf on certain Objections to the 



of the dependence of the refractive index upon the density of 

 the body is not absolutely exact. To this objection it must, 

 however, be replied that this chapter ( f the Variation of the Den- 

 sity with increasing Temperature ; ) is not by itself sufficient to 

 determine the correctness or incorrectness of a function : the 

 variations of the density of the body and its index of refraction 

 are nearly of the same order; consequently the most diverse 

 formulae may be proposed, which may give almost the same 

 results : but reference must be made to the results of the fol- 

 lowing chapter/'' 



My expression at this place, that, in spite of the recognized 

 diminution of M, the refractive index remained constant, is ex- 

 plained in the sentence (Studies, p. 63), " that the forces X w re- 

 tarding the light are functions of the temperature, and diminish 

 with its increase." 



In fact, I have (1865, 1866) derived the following successive 

 formulae for the indices of refraction. 



M «=1+VDX„^, (I.) 



^=l=ZGX/^ = M=f:. . . . (II.) 



^=1 + ZGD[M + C(£fl;. . . . (III.) 



in which — 



(j, is the index of refraction, 



V is the volume, 



D is the density, 



Z, G are the number and magnitude of the optical atoms, 



M is the refractive power, 



N is the dispersive power, 



P is the chemical equivalent, 



3.)? is the Newtonian equivalent of refraction, 



X w is a symmetrical function of the forces retarding the light 



in the body, 

 v 2 , s 2 } X 2 are the constants of the phase of the incident light. 



The constancy of the refractive power rests accordingly upon 

 the atomic arrangement of the molecules and upon the constancy 

 of the retarding forces X„. That the latter diminish with in- 

 crease of temperature, I have, as above stated, already mentioned 

 in my "Studies" (1866), and I examined the matter more closely 

 as soon as an opportunity presented itself for treating conducted 

 heat. 



In my text-book (finished in the autumn of 1867) the Ange- 



