WHEN EXPOSED TO POLARIZED LIGHT. 
51 
after many ineffectual attempts to obtain such a system, I succeeded by laying a very 
small portion of oil of laurel upon water placed in a black vessel, or on the surface of 
diluted or real ink. The rings thus produced are splendid beyond description, and 
exhibit the various phenomena with singular beauty. As the polarizing angle of 
the oil exceeds that of the water, the black - centred rings are seen at the polarizing 
angle of the water, when the reflected light disappears. They continue to be seen till 
we reach the polarizing angle of the oil, when the rings disappear, and the white-cen- 
tred ones commence, and continue till we reach the incidence of 90° *. 
In forming thin films upon metallic surfaces, I employed many of the metals, and 
found the phenomena nearly the same upon them all, and differing very little from 
those produced upon transparent bodies. On a fine specimen of specular iron ore, I 
found a system of rings ready formed, with three orders of colours. The azimuth of 
the polarized light being inclined 90° to the plane of reflexion, the system of rings 
disappeared wholly at an angle of incidence of 58° 36', which is therefore the polar- 
izing angle of the unknown substance of which it was formed : consequently its 
index of refraction is about T638. Between this angle and 90° of incidence, the 
white - centred rings appeared ; but at 7 2° 39', the polarizing angle of the iron (which 
gives its refractive power for the red rays 3*200), the rings were singularly fine, 
being seen on a beautiful blue ground, produced by the disappearance of the red 
light, which is polarized at that angle. I now measured the azimuth of the plane of 
polarization when the rings disappeared, which was 59° 25', whereas by the formula 
it is 57° 59' ; a discrepancy not to be wondered at, when we consider that the 
index of refraction for the red rays, viz. 3*200, was used, in place of that for the mean 
ray, which is not known. The inclination of the planes of polarization of the two 
interfering pencils, when calculated by the previous formulae, is + 32° 7', and — 5 7° 
53'; so that these planes being inclined 90° to each other, as in the case of soap and 
diamond, no interference takes place, and the rings disappear. 
In the fine specimens of oligist iron ore from Elba, l have found crystals covered 
with the most beautiful coloured films, both of uniform and variable thickness. These 
films are not acted upon by the ordinary acids, like the coloured films upon steel, and 
appear, from their optical properties, to be of a metallic nature. When they are ex- 
posed to a polarized ray, they exhibit generally the same phenomena as the films 
already described ; but there is no angle of incidence at which the colours disappear, 
either in the azimuth of 90°, at the polarizing angle of the first surface of the film, 
* These thin plates of oil of laurel exhibit some curious phenomena, which I believe have not been noticed. 
If we wet with water, alcohol, or the oil of laurel itself, the extremity of a short piece of wire, such as a large 
pin, and hold the pin in the hand, so that its head may be above, and almost touching the film, the film will 
recede in little waves of a circular shape, which form a new system of coloured rings ; and they become covered 
with the vapour from the fluid on the head of the pin in such small particles that they reflect no light, and the 
rings appear to be blackened. By withdrawing the pin, the film is restored to its former state. The same 
effect is produced by heating the pin, or the fluid upon it, to promote evaporation. 
H 2 
