I 
48 SIR DAVID BREWSTER ON THE PHENOMENA OF THIN PLATES 
dences between 53° 11' and 90°, in the case of the iriscope, very interesting- pheno- 
mena appear. We shall first describe what takes place at 56° 45', the polarizing angle 
of the black glass. At this angle none of the polarized light is reflected when the 
azimuth is 90°, and the rings with the white circumference are beautifully seen on 
the dark ground of the glass, which now reflects no light. As the azimuth is changed 
to 87°, 88°, &c., the black glass reflects a little light, and the two surfaces of the film 
a little more light, the rings gradually become fainter and fainter, till at an azimuth 
of about 79° 0' they disappear exactly as they did at 53° 11', and in the azimuth 90°. 
When this disappearance takes place, the light reflected from the glass seems to be 
exactly equal to the light reflected from both surfaces of the film. At other angles 
of incidence the rings disappeared at different azimuths, varying from 90° to about 
45°, as the angle of incidence varied from 53° 1 1' to 90°. I found it difficult, however, 
to measure these azimuths with any accuracy, as the rings were not permanent ; and 
I was therefore obliged to form the colours of thin plates upon highly refracting sub- 
stances, such as diamond, chromate of lead, artificial realgar , and greenockite (the 
most refractive of all bodies), which had high polarizing angles. A solution of fine 
soap gave brilliant colours when dried, and in this way I obtained the following re- 
sults with the surface of a very fine diamond. The index of refraction of the soap 
was 1'475, and that of the diamond 2’44, and their respective polarizing angles 55° 
52', and 67° 43'. 
Angle of Incidence of the 
Polarized Light. 
Azimuth of the Plane of Polarization at which 
disappear. 
Observed. Calculated. 
55 52 
o / 
90 0 
o t 
90 0 
60 
73 0 
74 27 
65 
68 30 
67 49 
67 43 
66 20 
65 10 
70 
63 30 
63 14 
75 
59 15 
58 23 
90 
46 30 
As the disappearance of the rings was not owing to the extinction of one of the 
interfering pencils, as at 55° 52', for a sufficient quantity of polarized light was re- 
flected from both surfaces of the film, there was reason to believe that it might arise 
from the two pencils being polarized at right angles to each other, in conformity 
with the law relating to the action of the second surfaces of plates which I have 
given in a former paper*. 
Calling# the azimuth of primitive polarization,* the angle of incidence on the first 
surface of the film, i! the corresponding angle of refraction, and consequently the 
angle of incidence on the second surface, i" the angle of refraction at the second surface. 
* Philosophical Transactions, 1830, pp. 148, 149. 
