t32 //. /'.'. Mii-triii Standards of Density. 



glass. 11 The indices of refraction of the glasses had been care- 

 fully determined on a standardized spectrometer, with a prob- 

 able error of less than do "00005. The index of refraction of 

 air was taken as unity, and of water at 20°, 1\3330. 



The five quartzes gave the following indices of refraction 

 for sodium light: Herkimer Co. and Brazilian pebble 1*5443-, 

 three others 1-5442. By the spectrometer the value found for 

 the Herkimer Co. quartz was 1-54428 ±'00004. 



G-ifford" has made most precise measurements of the indices 

 of refraction of quartz. His values for a> are very close to the 

 values obtained by most observers. The indices of refraction 

 of various quartzes, with rare exceptions, do not depart more 

 than ± -0001 from his values, which are given here {Li and 

 Tl are interpolated bv the writer). Z* = l - 5415 ; C = l - 5419 + ; 

 D = l-54425-; 77=1-5465 ; E = l-5472-; F = l"5497-. Accord- 

 ing to Gifford and others a rise in temperature of 17° to 20° 

 lowers the refractive index -0001. 



The refractive index of fluorite is an exceedingly constant 

 quantity ; even the colored varieties 13 differ from the clear 

 varieties scarcely more than db*0002. From the data in 

 Dufet's tables (pp. 436-439) and in later papers 14 the following 

 values have been compiled as representing the index of refrac- 

 tion of fluorite at 20°. The probable error in applying these 

 values to any clear fluorite is less than ±"0001. 



B 1-4320 Tl 1-4353 



Li 1-4322 E 1-4355 



C 1-4325 F... 1-4370 



D 1-43385 



8° to 10° rise in temperature causes a decrease of -0001 in 

 the refractive index. 13 



Plates of fluorite from Rosiclare, 111., Macomb, 1ST. Y., and 



Muscalonge Lake, N. Y., polished first on block tin then on 



felt with rouge, gave the following extreme values with sodium 



light, each the mean of sixteen readings of the refractometer : 



1-4338+ and 1-4338-. 



Geophysical Laboratory, Carnegie Institution of Washington, 

 Washington, D. C, Oct. 2, 1911. 



11 The refractive index of the glass hemisphere for sodium light calculated 

 from the reflection angles observed from these standard substances was : 

 from air l - 9062, water l - 9062, crown glass l - 9064, flint glass l - 9065, calcium- 

 magnesium silicate glass 1"9065. Water was most satisfactorily used by 

 placing a large drop on the hemisphere and covering it with a glass slip to 

 prevent cooling by evaporation. One edge of the glass slip was slightly ele- 

 vated to avoid diffraction bands by placing a bit of soft paper between the 

 glass and the hemisphere. A standard thermometer near the hemisphere 

 gave the temperature of the water. It was necessary to screen the hemi- 

 sphere from the direct radiation of the flame by a double glass screen, or to 

 place the instrument at a distance from the flame and take the light from a 

 large condensing lens. 



18 Proc. Eoy. Soc, (A) Ixx, 329, 1902. '« See ref. 8. 



14 F. F. Martens, Ann. d. Phys. (4F), viii, 459-465, 1902. See also refs. 6 

 and 12. 



