crystallized surfaces upon light. 1 53 
and having inclined the prism at a very small angle to that 
surface, I thus separated the image formed at the common 
surface of the prism, and the oil from the image formed at 
the common surface of the oil and the spar. The effect was 
exactly what I had anticipated. The influence of the ordinary 
reflecting force was reduced almost to nothing, and the light 
reflected from the separating surface of the oil and the spar, 
was polarised at an angle of about 45^®,and was almost entirely 
under the dominion of the force which emanated from the 
axis. The following were the results obtained with an ordi- 
nary surface, inclined 45° 23--' to the axis. 
1. Azimuth o°. When the plane of the principal section is 
in the plane of reflection, the light reflected at the surface of 
the oil and the spar is polarised in the plane of reflection, the 
obtuse solid angle being farthest from the eye. The light of 
the image is of a faint red colour, and has very little intensity. 
2. Azimuth 12 0 . The obtuse angle being farthest from the 
eye, the reflected pencil is polarised about 45 0 out of the 
plane of reflection. 
3. Azimuth 42°. The reflected pencil is polarised trans- 
verse to the plane of reflection, or 90° out of it. The light 
is now of a yellowish white tint, and is much more intense 
than in azimuth o°. 
4. Azimuth 90°. When the plane of reflection is perpen- 
dicular to the plane of the principal section, the obtuse solid 
angle being either to the right or left hand, the reflected 
pencil is polarised a little more than 135°, or — 43° out of the 
plane of reflection. The intensity of the pencil is now inter- 
mediate between that of azimuth o° and 45 0 . 
5. Azimuth 180°. The obtuse angle being now next the 
MDCCCXIX. X 
