﻿940 



Prof. C. Barus on the 



equal values of sin <£' is x 2 — x^ from which the wave-length 

 X is computed. This datum in turn gives the appropriate 

 (known) value N o£ the index of the liquid, all at 20°, so that 

 finally 



n =N sine//. 



4. Examples. Liquids. — The circle of the Kohlrausch 

 total reflectometer is only a little over 3 inches in diameter, 

 and the reading is just within *1 degree of arc. The index 

 of carbon disulphide moreover varies, relatively speaking, in 

 marked degree with temperature; i. e., "0008 per degree C. 

 Unless unusual precautions are taken, therefore, one cannot 

 expect the units of the third place to be quite trustworthy, as 

 it corresponds to about '07° of arc. The apparatus in my 

 possession was an old instrument, and a high order of precision 

 was of secondary interest, and not attempted. 



I will first take the case of an air plate with carbon 

 disulphide. A film of air was enclosed between two small 

 plates of microscope cover-glass by cementing them at their 

 edges with glue. In another case, plates of glass *3 cm. thick 

 were similarly prepared. An example of the results is given 

 in the curves" fig. 2, which show the value of N 20 in terms of x. 



Fisrs. 2 & 3. 



1-m 







0^ 



















tM 







/ 





















ir,r 





\ 



/ 





% 















| 



m 





\ 



K 





\ 

















ffr 





/ 





s. 



\ 



\ 

















m 



A 



/ 







< 

















m 



si- 



/ 



/ 



I 





Z 









^ 



t-A 



' O 







hi 



°7 

















^4 





9, 



m 



%=7 



8b 9 



I 



o m is 



o tfp\ ( 



:m. 









^i 



1 



0Vi^4 



4 A 



6 4 



i 5 



b 



a I 



4- 5 



6 5 



s e 



6 



I 6 



4- 65 



These curves should be quite symmetrical to the vertical, so 

 that for any height, N 20 , {x 1 + x 2 )/2 is constant. 



Similarly a given horizontal N 20 determines the values 

 # 2 — &\ for the computation of the wave-length X. The film of 



