Measurement of the Refractive Inde.r of Liquids. 301 



The special purpose of this note is to make clear that the 

 fifth fundamental equation of electromagnetic theory may be 

 derived from the four field equations and the principle of 

 relativity without making any arbitrary convention as to the 

 mass of a moving body. It is quite unnecessary to place the 



transverse mass of a moving bodv equal to — ° and the 

 longitudinal mass equal /-. _/o 2Vf- The simple relation for the 



mass of a moving body m= — ° , which was derived 

 J V1-/3 2 



directly from the principle of relativity by Lewis and Tolman 

 (loc. cit.) and from ideas of light pressure by Lewis (loc. cii.) 

 is sufficient. 



The fact that the fifth equation can be derived by com- 

 bining the principle of relativity with the four field equations 

 is one of the chief pieces of evidence which support the 

 theory of relativity. 



University of Michigan, 

 Ann Arbor, Mich., 



November 11, 1910. \ 



XXXVI. A Note on the Measurement of the Refractive Index 

 of Liquids. By 0. W. Griffith, E.Sc, A.R.C.S* 



FOR some years past the author has been setting his 

 students, as a laboratory exercise, to determine the 

 refractive index of water by using an ordinary spherical 

 flask filled with water as a convergent lens. The results 

 have always been strikingly concordant, the error in the 

 values obtained by different observers being small and fairly 

 constant. It was therefore thought that an inquiry into the 

 best conditions for the experiment might prove interesting, and 

 this paper contains the results of such an investigation and 

 indicates two very simple methods of determining the index 

 of refraction of liquids. It will be seen that these methods 

 are capable of giving accurate and reliable values. 



As a rule the problem of refraction through a sphere either 

 receives very meagre treatment in the ordinary text-book or 

 is relegated to the collection of mathematical exercises at the 

 end of the book. It is, however,, usually demonstrated that 

 the principal points of a sphere are coincident with its centre. 

 So that if U and V are the reciprocals of the distances of 

 conjugate foci measured from the centre of a transparent 



* Communicated bv the Author. 



