498 SEEBECK ON THE POLARIZATION OF LIGHT. 
this opportunity of communicating the measurements I have 
spoken of, which were made several years ago, and with the 
greatest possible care. Thus, I found the following values of 
this angle with the natural surface of fracture of the crystal, and 
one polished parallel to its axis, in different azimuths of the plane 
~of incidence. 
Azimuth, zr. On the natural surface. On that parallel with the axis. 
0 0 0 0 O 0 
22 30 +0 11 +2 43 
45 +0 23 +3 57 
67 30 —0O 48 +2 46 
90 —2 30 0 0 
112 30 —3 34 
135 —3 38 
157 30 —2 9 
180 0 0 
I have marked the examined angle with + when the plane of 
polarization was turned towards the same side as the principal 
section, when reversed with —. By the azimuth O and 180° that 
condition is understood in which the principal section is parallel 
with the plane of incidence, and in a natural surface 0, when the 
truncated extremity of the crystal is applied to the eye, 180° 
when it is turned towards the incident light. This difference 
vanishes in the case of the surface parallel to the prism, whilst 
from 90° to 180° the same recurs, but in reversed order. 
The results which are here detailed are averages of at least 
twenty observations, and those made upon the natural surface 
of fracture are possibly accurate to within a few minutes; this 
may be less frequent with the others, on account of the imper- 
fection of the polished surfaces ; still these do not differ aad 
from the true values. 
