52 
SIR DAVID BREWSTER ON THE PHENOMENA OF THIN PLATES 
or in those azimuths where the pencils, from the first and second surface, have their 
planes of polarization inclined 90° to each other. This, no doubt, arises from the 
high dispersive power of the film, in consequence of which the different homogeneous 
rays are polarized at angles considerably different from each other. 
The phenomena of transparent films of low refractive power, when laid upon the po- 
lished surfaces of metals, and exposed to polarized light, are not very different from those 
which are exhibited when the film rests upon a transparent surface. I at first used 
a solution of soap, which produced pretty good tints on speculum metal ; but at last I 
fell upon a method of laying down the most beautiful systems of coloured rings upon 
all surfaces of all forms, whether metallic, transparent, or opake. For this purpose 
I used the oil of laurel, which, when placed upon the surface of water, expands into a 
film, which gives the finest system of coloured rings. Having laid the plate of polished 
metal in a small porous wooden tray, such as is used for holding minerals, I poured 
water into it, so as to cover the metallic surface to the depth of the 50th part of an 
inch. I then formed a film of the oil upon the water, immediately above the metallic 
surface. In a short time the absorption of the water by the porous tray allowed the 
film of oil to descend and rest upon the metallic surface *. When the adhering 
moisture was removed by evaporation, the film was extremely beautiful ; and if pro- 
tected from dust may be preserved for any length of time. 
Having laid a film of this kind upon speculum metal, I obtained the following re- 
sults. The coloured rings disappeared almost completely at 56°, the polarizing angle 
of the oil. The black-centred rings appeared at all angles less than 56°, and the 
white-centred rings at all angles above it. Both the systems of rings were exceed- 
ingly distinct at the greatest angles of incidence, whereas on transparent surfaces of 
low refractive power, they can scarcely be seen at such angles. When the azimuth 
of the polarized ray varies from 90° to 0°, the rings disappear at different angles of 
incidence ; or when the angles of incidence vary, the rings disappear in different 
azimuths. I measured these azimuths when the polarized ray was incident upon 
speculum metal, and obtained the following numbers: — 
Azimuth in which the rings disappear. 
Angles of Incidence. 
Observed. 
Calculated. 
Difference. 
o / 
90 
O / 
40 23 
71 50 
56 25 
5 7 22 
- 0 57 
60 0 
65 45 
65 4 
+ 0 41 
56 5 
90 0 
90 0 
In computing column third from the formula in p. 49, I used T49 as the index of 
refraction of oil of laurel, and 4’011 as the index of refraction for speculum metal, as 
deduced from my experiments on its elliptic polarization ■'f-. 
* The same effect is produced more slowly by evaporation ; or the water may be sucked out of the tray by 
a tube, or run off by an aperture. 
f Philosophical Transactions, 1830, p. 324. 
