7i 
crystallized bodies on homogeneous light. 
means of putting this remarkable consequence to the test 
of experiment. In fact, in the three series of tints observed 
in sulphate of baryta, the apparent angles between the axes 
for the mean red rays are respectively 62° o', 62° 2', and 6i° 
53', the mean of which is 61*58', while the apparent angles 
between the virtual poles in the same series are 72 0 46', 72 0 
36', and 72°47 / . The semi-excesses of the latter angles over 
the mean value of the former, are the apparent angular 
distances of the virtual poles from the axes of mean red rays, 
and are respectively 5° 24', 5 0 19', 5 0 25', neither of which 
differs more than 4' from the mean. 
To ascertain the real value of 9 , we have only to compute 
the angles of refraction. In the specimen employed, I found 
1-6475 for the index of ordinary refraction, and the angles 
of incidence (the halves of the above angles) being 30° 59' 
and 36° 23', 36° 18', 36° 24' ; the corresponding angles of re- 
fraction are 18 0 i2'3o"( = a), 21 °6' 10", 21 0 3' 40", 2i°6'4o"; 
and since 9 = q> — a in this case, we find for the values of 9 , 
2 0 52 > 4° f/ 5 2° 5 1 ' 10", 2 0 54' 10", the mean of which gives 2 0 
53' o" for the real angular distance of the virtual pole from 
the axis of mean red rays 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' = 19° 33'; 28 1° 30' — 262° o' = 19 0 30', 
and 282° o' — 262° 25' = 19 0 35' 
of which neither differs more than 3' from the mean 19 0 33'. 
Dr. Brewster (Phil. Trans. 1814, p. 216) has stated the 
refractive index of this salt at 1*515 ; but this is certainly a 
