56 



Sir Joliii Conroy on the 



[May 18, 



imposed, and so placed with respect to each other that they transmitted 

 the maximum amount of light. 



The light of a paraffine-lamp reflected from the surface of a film, con- 

 sisting partly of iodine a and partly of iodine p between two slips of 

 glass, was examined by means of the Nicol, the angle of incidence being 

 about 60°. 



When the principal section of the Xicol was in the plane of incidence, 

 and when consequently but little of the light reflected from the surface 

 of the glass was transmitted, portions of the film of iodine appeared of 

 different degrees of brightness ; and on rotating either the Nicol or the 

 stand, the relative brilliancy of different portions of the film changed, 

 those portions which consisted of iodine /3 appearing perfectly black in 

 certain relative positions of the film and Nicol, whilst the remainder of 

 the film merely became more or less bright. 



The film of iodine was then placed so that the portion consisting of 

 iodine /3 appeared perfectly black w^hen the principal section of the Nicol 

 was in the plane of incidence. On rotating the stand, light reflected from 

 the surface of the iodine was transmitted by the Nicol, and increased in 

 quantity till the stand had been turned through 90°, when the surface 

 of the iodine had a brilliant metallic lustre. On continuing the rotation, 

 the surface gradually lost its brilliancy, and Avhen the stage had been 

 turned through 180° appeared perfectly black again. 



On rotating the Kicol the same changes took place ; but the light 

 reflected from the surface of the glass marked the eifect to a consider- 

 able extent when the principal section of the Kicol was no longer in the 

 plane of incidence. 



The light incident upon the surface of the glass showed no signs of 

 polarization when examined by a double -image prism and plate of sele- 

 nite, and only the faintest trace of it after passing through, at an angle 

 of about 60°, a slip of glass similar to those used for covering the layers 

 of iodine ; consequently the polarization of the light must be due to the 

 film of iodine. 



From this it appears that the light reflected from the surface of a 

 film of iodine /3 is polarized ; and b}'' examining the light transmitted by 

 the same portion of the film, it was ascertained that the plane of polari- 

 zation of the reflected light is perpendicular to that of the ray w^hich is 

 most freely transmitted, and consequently that the reflected light is 

 polarized in a plane at right angles to the axis of the crystals. 



"When the incident light was polarized, it was found that it was 

 reflected from the surface of the iodine when the plane of polarization 

 of the light was perpendicular to the axis of the crystals, and extinguished 

 when parallel. 



As has been stated before, when a film of iodine o is seen through a 

 Nicol, it does not appear black in any position ; but the brilliancy of the 

 surface alters as the Nicol or iodine is rotated. 



