\. 



324 W. T. Schaller — Refractive Index of Canada Balsam. 



Art. XXV. — The Refractive Index of Canada Balsam; by 

 Waldemae T. Schaller. 



The refractive index of Canada balsam, as it occurs in the 

 thin sections made for the U. S. Geological Survey, was 

 determined on the request of Mr. F. C. Calkins, who had 

 found* that the index, or n, was not absolutely constant but 

 varied between two extremes. By the examination of 300 

 slides, he found n to reach and even slightly exceed co of quartz 

 (1*54:4:), though n was found greater than 1-514 only in the 

 proportion of one slide in a hundred. The excess was very 

 small and the balsam was decidedly yellow. The lowest value 

 found by him was about 1*535 ± -002. 



The value of n for sodium light was determined on an Abbe- 

 Zeiss refractometer by total reflection on three kinds of slides, 

 which were (1) not cooked as much as usual, (2) cooked as 

 ordinarily done, and (3) over-cooked. The differences found 

 between (1) and (2) are very slight, and, in fact, the individual 

 values show almost as much variation as between the different 

 groups. The values obtained are : 



(1) 





(2) 



(3) 

 f 1-543 



1-539 





( 1-536 



| 1-540 



1-538 



n = 



: 1 1-538 



n = \ 1-540 



1-539 





( 1-539 



| 1-542 

 L 1-541 



The average values are for (1), 1-5387: for (2), 1*5377; for 

 (3), 1*5412, or, as the average of all, 1*5395, which is almost 

 identical with the value (1*5393) given by Beckerf in 1898. 

 A determination of n in a slide six years old gave the value 

 1*5390. These values show that, in general, n lies very close 

 to 1*539 and that this value may well be used in a study of a 

 thin section, while the actual possible variation was fonnd by 

 Mr. Calkins to be from 1*535 to 1*545, though the extreme 

 values are but seldom reached. The uncooked, liquid balsam 

 has a refractive index of 1*524, which, after cooking, rises to 

 1-54. The older a slide, the higher the index of the balsam 

 becomes, which after a time, especially if the air has access, 

 reaches towards the highest value, or 1*545. 



Chemical Laboratory, U. S. Geological Survey. 



* Science, vol. xxx, p. 973, Dec. 31, 1909. 

 fThis Journal [4], v, p. 349. 



