e = 



MERAVI>7: MEASUREMENT OF REFRACTIVE INDEX 533 



o) sin jS 



(5) 



(6) 



co- 

 sin /3 



^ V^ co2 eco/ 



This is a true equation for the extraordinary refractive index of a 

 uniaxial crystaL- 



From measurements of 13 and I on two rings of the same sec- 

 tion, equations can be formed from (6) which when solved 

 simultaneously permit the elimination of one of the constants 

 0), e, X, e. However, the resulting equations have no practical 

 application, for verj^ slight errors in the observed values of ^ 

 may make very large errors in the values calculated. 



If CO, e and X are known e, the thickness of the plate, is readily 

 and accurately found thus: 



By transposition (4) becomes ei = -J co^ + sin^ /3 ( 1 — — ) 

 Then by substituting (3a) 



From the idea of the isochromatic surface several roughly 

 approximate formulas^ have been developed which contain a 

 term in which the double refraction appears. These assume 



- This equation is identical with equation g (expressed in the form of equation 

 6) of the article by F. E. Wright in this issue; the present equation is derived, 

 however, by a different method. 



» See Preston's The Theory of Light, pp. 393-345; and Duparc and Pearce's 

 Traite de Technique Alineralogique et Petrographique, pp. 312-313. 



