150 Dr. Brewster on the action of 
passes through the rest of the crystal, and is polarised in a 
transverse plain.* 
Sect. II. On the influence of the polarising force of doubly re- 
fracting crystals , upon the polarising force which accompanies 
partial reflection. 
The experiments in the preceding section could not fail to 
throw a doubt upon the identity of action exercised upon re- 
flected light by crystallized and uncrystallized surfaces. I 
was therefore led to a more minute investigation of the sub- 
ject, and obtained a series of very unexpected results, which 
I shall explain under the three heads into which they naturally 
arrange themselves. 
1. On the change produced upon the polarising angle by the 
interior forces of doubly refracting crystals. 
In order to examine with care the superficial action of cal- 
careous spar, I exposed several surfaces by cleavage, and 
having selected the one that had the most perfect polish, I 
covered all the other sides of the rhomb with black wax, and 
measured the polarising angles in planes variously inclined to 
the principal section. The following are the results of a great 
number of observations : 
Position of the crystal. Azimuth. 
Short diagonal in plane of reflection o° 
Polarising No. of 
angle, observations. 
57 ° * 4 / 39 
One of the edges in plane of reflection 50 £7§-' 58 32 15 
Long diagonal in plane of reflection 90 59 32 37 
Difference between the greatest and least po- 
larising angle - - - - 2 0 18' 
* See Phil. Trans. 18x3, p. 104; 1814, p. *91 ; and Edin. Trans, vol. vii. p. 298, 9. 
