268 Profs. Liveing and Dewar on the Refraction 



however, equally applicable to the measurement of the differ- 

 ence of potential between the terminals of an inductive resist- 

 ance, and has in fact been used bv us for such measurements. 

 The difference of potential (v) in such a case is given in terms 

 of the observed quantity (e) by the relation 



1 de 



v=e+- 



where 



V=e+ V^ 



p 2 



9+P-- 



a!— £L. 



a formula which is derived by reasoning similar to that 

 employed above. 



Physical Laboratory, 

 University College, London. 



XXI. On the lief raction and Dispersion of Liquid Oxygen, and 

 the Absorption Spectrum of Liquid Air. By Professors 

 Liveing and Dewar*. 



IN August 1892 we published in this Magazine (vol. xxxiv. 

 p. 208) a measure of the refractive index of liquid 

 oxygen at its boiling-point for the yellow sodium rays, made 

 by means of a prism. In the following year (vol. xxxvi. 

 p. 330) we published a measure of the same quantity made 

 by a different method. For the reasons stated, we could only 

 obtain measures more or less approximate to the truth. 

 Since then we have made several attempts, but hitherto in 

 vain, to make hollow prisms with vacuous jackets, in which 

 the liquid oxygen could be kept in a tranquil state while 

 the observations were going on. We have also attempted 

 unsuccessfully other methods of taking the measures. 



The chief difficulties which we encountered in making our 

 former measures arose from the irregularities and striations of 

 the glass vessels, and from the continual ebullition of the liquid 

 oxygen. These difficulties have now been to a great extent 

 obviated. We have come back to the method we used in 

 1893, which we then described as the method of MM. Terquem 

 and Trannin, but which had previously been suggested by 

 Prof. E. Wiedemann (Archives de Genhe,\\. p. 340, 1874). 

 However, for the cylindrical vessel before used we substituted 

 a globular vessel having the inside of its vacuous jacket 



* Communicated by the Authors. 



