WHEN EXPOSED TO POLARIZED LIGHT. 51 



after many ineffectual attempts to obtain such a system, 1 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 irm are, 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 1*638. Between this angle and 90° of incidence, the 

 white-centred rings appeared ; but at 72° 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 -f- 32° 7', and — 57° 

 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, I 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 

 eflfect is produced by heating the pin, or the fluid upon it, to promote evaporation. 



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