182 Dr. A. Schrauf on certain Objections to the 



is based upon the data of Biot and Arago, which are to the effect 

 that the variation of the indices of refraction is proportional to 

 the increase of density. They say, " La refraction d'un mc'me 

 gaz quelconque est toujours rigoureusement proportionelle a sa den- 

 site"*. Biot and Arago nevertheless decide to retain the New- 

 tonian refractive power, since for gases, as is well known, 

 ^-1 = 2(^-1), 



and hence the investigations on gases do not make manifest any 

 distinction between formulae (II.) and (VI.). 



Beer, in his 'Introduction to Higher Optics ' (p. 36), has 

 again brought forward this formula (VI.). In a similar manner 

 it has been recently employed by Dale and Gladstone, Landolt 

 and Haagen. 



In my ' Equivalents of Refraction ' (1865) I have submitted 

 this formula to an examination. To distinguish it from New- 

 ton's refractive power, I called formula (VI.) Biotas refractive 

 power, and denoted it by the letter m. In the same manner I 

 determined for a great number of substances, not only the New- 

 tonian refraction-equivalents 2)?, but also those values correspond- 



ing to the formula 



(*) 



Pm = m (VII.) 



These values may also claim to be considered refraction-equiva- 

 lents. I distinguish them by the name "Biot's refraction-equi- 

 valents " (formula VII.). These latter values, however, in accord- 

 ance with chemical usage, may be called " little refraction-equi- 

 valents," since invariably J)? > \\\. 



The values of $? relate to a unit such that $?(H) = 0"00-1. 

 In order to facilitate a comparison of the numerical values, I in- 

 troduced another unit which is so chosen that [2)f] (H) = 1*0, and 

 I call the value of [5)?] the reduced refraction-equivalent. In 

 these last-mentioned values, which arc simply relative, we get 

 rid of every assumption concerning the unit of density used. 

 The other values of 5)? and $1 are calculated on the base of the 

 density of the air being unity. To convert these into numbers 

 corresponding to the density of water considered as unity, we 

 have merely to multiply by 773*45. 



From what has been above said, it is very clear that I have 

 attached proper importance to Biot's formula (VI.). But yet I 

 am still compelled to profess my adherence to Newton's refrac- 

 tive power. 



The reasons for this are in part theoretical and in part prac- 

 tical. I shall mention several in the sequel. 



* Mem. de VInst. Paris, 1806, vol. i. p. 322. 



