ivheti exposed to Polarized LigJit. 191 



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 oi' oligisi 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 exposed to a polarized ray, 

 they exhibit generally the same phfenomena as the films 

 aheady described ; but there is no angle of incidence at which 

 the colours disappear, either in the azimuth of 90°, at the po- 

 Lirizing angle of the first surface of the film, or in those azi- 

 muths 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 phaenomena of transparent films of low refractive power, 

 when laid upon the polished 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^ wliich, when 

 placed u})on the surface of water, expands into a fihn, 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 fiftieth part 

 of an inch. I then formed a film of the oil upon the water, 

 iuimediately 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 ujjon the metallic surface*. When 

 the adhering moisture was removed liy evaporation, the film 

 was extremely beautiful ; and if protected from dust may be 

 preserved for any length of time. 



Having laid a film of this kind upon speculum mclal, I ob- 

 tained the following results. The coloured rings tlisappeared 

 almost completely at 5G", the polarizing angle of the oil. The 

 black- centred rings appeared at all angles less than b(f, and 

 the xthitc-cenlrcd rings at all angles above it. Both the sy- 

 stems of rings were exceedingly distinct at the greatest angles 



* The hjuiic effect is produced more slowly by evaporation ; or tlic water 

 iiiny be sucked out of the tray by a tube, or run off by an aperture. 



