152 
Dr. Brewster on the action of 
to the axis ; but the results clearly proved that the peculiar 
action of the surfaces, in varying the polarising angle, is ex- 
hibited only by the highly polished faces which are sometimes 
obtained from cleavage, or which occur in perfect crystals. 
The following observations were made with a fine crystal 
of Chromate of lead : 
Position of the crystal. Polarising angle. No. of observations. 
Axis of prism in plane of reflection 67° 48' 4 
Axis of prism perpendicular to the 
plane of reflection - 65 42 4 
In the first of these positions a great quantity of brilliant 
blue light remained unpolarised, whereas in the second posi- 
tion the whole of the pencil suffered complete polarisation. 
2. On the change produced upo?i the polarisation of the reflected 
ray , by the interior forces of doubly refracting crystals. 
Since the extraordinary force in calcareous spar was thus 
shown to extend to such a distance beyond the surface as to 
modify the polarising angle produced by superficial reflection, 
it became extremely probable that the polarisation of the re- 
flected ray might suffer some change from the same cause : 
but after the most careful observation, I could not discover 
the slightest indication of such an effect. Upon reflecting 
farther, however, on the nature of the change which I had 
expected, it occurred to me that the action of the ordinary 
reflecting force was so powerful, as to mash the influence of 
the inferior force which emanated from the axis, and that the 
effect of the one might be rendered visible by diminishing the 
intensity of the other. I accordingly introduced a film of oil 
of Cassia between a glass prism and the surface of the spar, 
