102 Mr» Herschel on the Action of 



(the halves of the above angles) being 30° 59'' 



S6° 18' 



[March, 



and 36° 23^ 

 the corresponding angles of refraction are 



18° 12' 30'^ (= fl), 21° 6' 10'', 21° 3' 40", 21° 6' 40" ; and since 

 fl = ^ — a in this case, we find for the values of d, 2° 53' 40^% 

 2° 51' 10", 2^ 54' 10", the mean of which gives 2° 53' 0" for 

 the real angular distance of the virtual pole from the axis of 

 mean red rayi in sulphate of baryta. 



Again, in the series of tints, Tables V, VI, VII, for Rochelle 

 salt, the apparent angular distances of the mean red axis from 

 the virtual pole were 277° 8' ~ 257° 35' = IST 33'; 281° 30^ 

 — 262° 0' = 19° 30', and 282° 0' - 262° 25' = 19° 35', of 

 which neither differs more than 3' from the mean 19° 33'. 



Dr. Brewster (Phil. Trans. 1814, p. 216) has stated the 

 refractive index of this salt at 1-515 ; but this is certainly a httle 

 too large. In four experiments made at distant intervals of 

 time, and by different modes of observation, I have found 



1-49640 

 1-49670 



1-50220 

 1-49853 



for the index for the mean yellow-green rays, of which determi- 

 nations the last is to be preferred, having been made with great 

 care. The same experiment gave 1*49293 for the index for 

 mean red ray&. The apparent angular distance of the axis for 

 red rays from the perpendicular was 1 6°, which leaves 3° 33' 

 for the angular distance of the virtual pole from the perpendi- 

 cular. These angles of incidence correspond to the respective 

 angles of refraction 10° 38' 20" and 2° 22' 40", of which the 

 sum 13° 1' is the real angle between the virtual pole and mean 

 red axis in Rochelle salt. The series in Table IV. gives 13° 2' 

 25" for the value of Q, which agrees completely with the fore- 

 going determination. 



I took seven plates of nitre of various thicknesses, and cut 

 from different crystals, and by a mode of observation to be 

 described hereafter, found as follows : 



Table VIII. 



