of Light at Certain Metal-Liquid Surfaces. 99 



as often as might be necessary in the course of a set of observa- 

 tions. 



The phase difference is obtained from the difference of the 

 compensator readings of (3) and (4) and those of (1) and (5). 

 The double angle of restored polarization is the difference of 

 the positions of the analyzer in (3) and (4). 



There were made, in all, nine complete sets for A and 2-v/r as 

 described above : four in air, three in water, and one each in 

 the H.C1 and turpentine. The results are shown in the follow- 

 ing table. In the first column are given the observed values 

 of the displacement of the center fringe in terms of turns of 

 the compensator screw, and in the second the directly observed 

 values of 2ty (or 2\Jr o ). 



Comp. 2ip or 2t/> 



Air 0-7538 ± 0-0041 81-04° ± 0-35° 



" 0-7549 ± 0-0018 81-54 ±0-19 



" 0-7550 ± 0-0017 81-32 ± 054 



" 0-7548 ± 0-0052 81*46 ±0-23 



" (av.) 0-7546 ± 0-0017 81-34° ± 0-18° 



Water.. 0-9975 ± 0-0035 81-65 ±0*48° 



" 0-9935 ± 0-0010 82-05 ±0-35 



" 0-9934 ± 0-0039 81'74 ±0-20 



" (av.) 0*9948 ± 0*0020 81-81° ± 0-16 ° 



HC1 1-0030 ± 0-0099 81-91° ± 0-35° 



Turpentine.. 1-0035 ± 0*0069 82-08° ± 0-34 



From the average values above, the values of 77--A (or 7r-A ) 

 are computed by multiplication with the compensator constant, 

 while the values of Q (or Q ), P (or P ), S (or S ), 8, and n' 

 are calculated by equations (3), (6), and (8). The results of the 

 computations are given in the next table. 



A or A o Q or Q P or P S or S„ <5 n' 



Air 149-73° ± 0-07° 73-19° 148-63° 5-343 



Water 140-10° ± 0-08° 77-38° 139-40° 4-055 0-0084 1-329 ± 0-005 



HC1 139-73° ± 0-40° 77-58° 139-10° 4-024 0-0086 1-340 ± 0-015 



Turpentine 136-35° ± 0-28° 78-61° 135-78° 3-693 0-0127 1-465 ± 0-011 



Finally, samples of the liquids used were put in a hollow 

 prism and their indices determined in the usual way from the 

 angle of the prism and the angle of minimum deviation. The 

 values thus obtained for n were : 



WO 



Water 1*3320 



HC1... 1-3375 



Turpentine 1*4690 



These values are certainly correct to the third place. 



