crystallized surfaces upon light. 155 
I have repeated the preceding experiments by substituting 
in place of oil of cassia, water, alcohol , castor oil, balsam of 
capivi, and oil of anise seeds, a series of fluids whose refractive 
powers increase progressively. With water, the light refuses 
to be polarised completely in the direction of the long diago- 
nal, while it suffers complete polarisation in the direction of 
the short diagonal. With alcohol, the direction of the polari- 
sation is not altered. With castor oil, the intensity of the light 
is greater in the direction of the long diagonal, than in that 
of the short one ; and in the former case, the pencil is pola- 
rised at a much greater angle than in the latter. With balsam 
of capivi, in the azimuth of 45 0 , the pencil is polarised about 
15 0 out of the plane of reflection. In the azimuth of 90°, the 
pencil is not completely polarised at any angle, but is nearly 
so in the plane of reflection, and at a considerable angle of 
incidence. In o° of azimuth, the pencil is completely polarised 
in the plane of reflection. With oil of anise seeds, in azimuth 
45 0 , the pencil is polarised about 45 0 out of the plane of re- 
flection. In azimuth go 0 , the pencil refuses to be polarised 
at any angle, and in o°of azimuth, the polarisation is complete 
in the plane of reflection. 
As the preceding results were obtained with a surface in- 
clined 45 0 23-j' to the axis, I was anxious to observe the 
effects produced by the Faroe cr} T stals, where the natural 
faces are nearly in the plane of the axis. I accordingly 
•repeated the experiments with a variety of these crystals, and 
in every case I observed the same phenomena. In the azi- 
muth of go°, where the polarising angle is 58° 14', the pencil 
was polarised a degree or two out of the plane of reflection. 
In the azimuth of 45 0 , where the polarising angle is about 
