274 E. S. Larsen — Relation between the Refractive Index, etc. 



great as —11 percent, as in CaO. The percentage of differ- 

 ence for the n 2 formula may likewise be positive or negative 

 and reaches a value of 9*8 per cent for CaO. 



Summary. 



A study of the refractive indices and densities of silicate 

 glasses and of artificial minerals was made to test the formulae 

 of Gladstone and Dale and of Lorentz and Lorenz. Neither 

 the refractive indices nor the specific volumes of the glasses 

 are strictly additive functions, but there may be an increase or 

 a decrease of volume and a corresponding decrease or increase 

 of the refractive index. The specific refractivity computed 

 from either formula is sensibly additive for the glasses and 

 nearly so for the isomorphous series of soda-lime feldspars, 

 but when crystals are compared with glasses of the same 

 composition _, or with other crystals, the values of the specific 

 refractivity, computed from either formula, may differ by as 

 much as eleven per cent. They are usually higher for the 

 glasses. One formula appears to hold as well as the other, but 

 the formula of Gladstone and Dale has the advantage of 

 simplicity. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 



Washington, L>. C, May, 1909. 



