Theory of the Equivalent of Refraction. 181 



To conclude my proof, I must consider yet another possible 

 objection — namely, it is possible that we might wish to make 

 use of a second hypothesis for the explanation of the secondary 

 abnormal variations of the refractive power, and to" derive these, 

 not from the variation of X„, but from an alteration in the num- 

 ber and magnitude of the atoms, making use of the equation 

 M = ZGX t ,. I myself should assent to such an explanation if it were 

 possible to reconcile an alteration of the atoms with the hitherto 

 conceived notion of conducted heat. The nature of heat compels 

 us rather, as before, to consider length and rate in relation to 

 the paths of the molecules. 



The above developments are essentially based upon the Studies 

 published by me at the end of the year 1866. As the latter 

 contain so many improvements and additions to my former pub- 

 lications, I cannot discuss the latter. They fulfilled their purpose 

 of directing the attention of physicists to a hitherto neglected 

 field, and they started the conviction that we cannot, as was for- 

 merly the case, regard light and matter as being without direct 

 mutual relations. 



The relations and fundamental propositions first advanced by 

 me (1862) I have entirely reconsidered, as just mentioned, in 

 my work f Equivalents of Refraction/ ] 865, and f Studies/ 1866. 

 I have even retracted in the latter publications many of my 

 former conclusions. This is notably the case with my former 



formula,^ (compare formula II.). Accordingly I consider it 



superfluous to reply to M. Ruhlmaun's remarks on this subject; 

 for I can only explain M. Ruhlmann's remarks by supposing 

 that he neglected my later publications, or was ignorant of their 

 existence. 



The dispersive power adopted* by me in my " Studies/' for- 

 mula (II.), shows no well-marked relation to temperature ; and as 

 far as the observations hitherto extend, the variations of the dis- 

 persive power are, in liquid bodies, partly positive, partly negative, 

 in relation to the normal value. 



Having in the above paragraphs endeavoured to place the ob- 

 jections against my theory in their true light, I must in this 

 place call attention to certain further facts in confirmation of 

 Newton's equivalent of refraction. 



Of late a second formula has been advanced to represent the 

 relation between density and index of refraction. This formula, 



— yc— =m— constant, (VI.) 



* PN= 9t is called the equivalent of dispersion. 



