58 



Sir Joliii Conroy on the 



[May 18, 



wHen the axes of the crystals are parallel ^^'ith the plane of incidence ; 

 consequently the appearance of the film is exactly the same when un- 

 polarized light falls on its surface and it is seen through a Mcol whose 

 principal section is vertical, and when the incident light is polarized 

 perpendicularly to the plane of incidence and it is seen directly. 



These experiments show that when light falls upon the surface of a 

 film of iodine at an angle of about 72° a portion of the light is polarized 

 by reflection in the plane of incidence, and this independently of the 

 position of the crystals composing the film, and tliat another portion of 

 the light, which is coloured by reflection, and to which the surface-colour 

 is due, is polarized in a plane whose direction depends on that of the 

 crystals composing the film, and, further, that this light is polarized 

 perpendicularly to the axis of the crystals. 



The surface-coloiu' can only be seen when the angle of incidence 

 which the light makes with the surface of the iodine is a large one ; 

 and the reason that in the case of iodine covered vtith glass it is not 

 risible, apparently is, that with a large angle of incidence nearly the 

 whole of the light is reflected from the surface of the glass. I succeeded 

 in seeing the blue colour' in the case of a fragment of iodine which had 

 been melted between a slip of glass and one of the sides of a small cro^^^l- 

 glass prism of an equilateral section, and also when such a prism was 

 placed on the surface of one of the glass slips covering the iodine, a 

 drop of carbon tetrachloride (the index of refraction of this liquid being 

 nearly the same as that of the glass) being placed between the slip and 

 the prism, as under these circumstances light can reach the surface of 

 the iodine at a greater angle than is possible when it is covered by a flat 

 piece of glass ; but in neither case was the surface-colour so well seen 

 as when the iodine was uncovered. 



Haidinger has remarked (Sitzungsberichte der kaiserlichen Akade- 

 mie der Wissenschaften, Band ^-iii. p. 97) that the surface-colours are 

 complementary to the colour of the light transmitted by the same 

 substance ; and this also appears to be the case with iodine, as in the 

 solid a,nd liquid condition, and also when dissolved in certain liquids, it 

 absorbs most readily the blue rays ; but at the same time, as Professor 

 Stokes has pointed out, the sm'face-coloiu' and the colour of the trans- 

 mitted Kght can only be said to be complementary within very narrow 

 limits, as the colour of the transmitted light varies with the thickness of 

 the layer of substance through which it passes. 



Films of iodine a and /3, when the light was incident on their surface 

 at a considerable angle, were found to polarize the light elliptically in 

 certain positions of the film, as was shown by the black cross being dis- 

 torted when a plate of Iceland spar, cut perpendicularly to the axis 

 of the crystal, and a plano-convex lens of about 40 mm. focal length 

 were placed between the surface of the iodine and the analyzing Mcol. 

 The amount of distortion, which was never very considerable, increased 



