/Standards of Density and Refractive Index. 431 



It is of course essential that fiuorite or quartz to be used as 

 standards of density should be clear. The specimens should 

 be carefully examined in alcohol or water under the microscope 

 in order to detect minute cracks or inclusions. 



By means of water, and solutions standardized by quartz and 

 fiuorite, the Westphal type of balance of standard make can 

 easily be adjusted to an accuracy of ±'0015 for all readings. 



Substances of standard index of refraction are useful chiefly 

 for adjusting, or determining the errors of the total refractom- 

 eter. If the indices of refraction of the glass hemisphere for 

 certain wave lengths are not known, the} 7 may be calculated 

 from the value of the observed angle of total reflection of these 

 wave lengths from the surface of the standard of known index 

 of refraction. If N is the index of refraction of the hemi- 

 sphere, n that of the standard, and a the observed angle, 



then -JL_ = N. 

 sin ex 



The refractive index of quartz has been the subject of 

 repeated investigations. Most of the careful and experienced 

 observers have obtained values that are accordant within 

 ± - 0001. The extreme values given in Dufet's tables 

 (pp. 427-432) for the ordinary ray and the sodium lines are 

 1-5446 and T544L2. Reference to the original papers shows 

 that few of the observers have stated the locality from which 

 their quartz was obtained. For this reason it cannot well be 

 determined whether the index of refraction varies to this 

 extent or whether the values obtained are in error. Essel- 

 bach, 8 who obtained the highest value, considered the possi- 

 bilities of error and concluded that the high values he obtained 

 throughout the entire spectrum were probably due to the 

 quartz. Danker 9 obtained 1'5444 for quartz from Middleville, 

 Herkimer Co., New York, by total reflection. A comparative 

 study by the writer of the index of refraction for sodium light 

 of five quartzes, one from Herkimer Co., New York, and one 

 of Brazilian pebble, gave, by the Abbe-Pulfrich total refractom- 

 eter, an extreme difference 10 of '00015. Each value obtained 

 represents the average of 32 readings (16 for each of two 

 polished faces) on both parts of the graduated circle. The 

 highest index was obtained from the Herkimer Co. quartz, and 

 the lowest from the lightest quartz used in the tests for density. 

 The accuracy of the adjustment of the refractometer and the 

 errors of its vernier and circle were determined by observing 

 the angle of total reflection from air, water at known tempera- 

 tures, crown glass, flint glass, and a calcium-magnesium silicate 



8 Pogg. Ann., xcviii, 541-540, 1856. 



9 N. Jahrb. f. Min., Beil.-Bd. iv, 266, 1886. 



10 Dufet (Bull. Min. Soe. de Fi\, xiii, 274, 1890) found about this difference 

 among twenty quartzes — some deeply colored. 



