190 Sir D. Brewster o?i the Pheendmena of Thin Plates 



feet system of rings when the film had a greater refractive 

 power than the substance upon which it was laid ; 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 ex- 

 hibit the various phiienomena with singular beaut)'. 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-centred ones commence, and 

 continue till we reach the incidence of 90°*. 



In forming thin films upon metallic surfaces, I employed 

 many of tiie metals, and found the phaenomena nearly the 

 same upon them all, and differing very little from those pro- 

 duced upon transparent bodies. On a fine specimen of .yje- 

 ciilar 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 wliolly at an angle of incidence of 58° 36', 

 which is therefore the polarizing angle of the unknown sub- 

 stance of which it was formed : consequendy its intlex of re- 

 fraction is about I "638. Between this angle and 90" of inci- 

 dence, the white- centred rings appeared ; but at 72' 39', the 

 polarizing angle of the iron (which gives its refractive power 

 for the 7-ed rays 3"200), the rings were singularly fine, being- 

 seen on a beautiful blue ground, produced by the disappear- 

 ance of the 7-ed 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 t!ie previous 

 formula?, is +32^ 7', and —57° 53'; so that these planes being 



* These tliin plates of oil of laurel exhibit some curious |)hocnon)eiia, 

 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 lai-ge 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 coveied with the 

 vapour from the fluid on the head of the piii in such small particles that 

 tiiev 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. 



