1876.] 



Polarization of Light, 



59 



with the angle of iiicicleuce, and appeared to attain its maximum when tho 

 angle was about 72°. 



When a film of either iodine a or /3 is placed on the stage with its 

 axis perpendicular to the plane of incidence, and the principal section of 

 the analyzer parallel with the latter plane, the black cross is perfect ; 

 but on turning the film till its axis is parallel with the plane of incidence, 

 the cross becomes slightly distorted, and the centre appears bluish. 

 When the light falling on the surface of the film is polarized in a plane 

 forming an angle of 45° vTith. the plane of incidence, the principal 

 section of the analyzing JSTicol still remaining in that plane, the black 

 cross is perfect as long as the axis of the iodine is perpendicular to the 

 plane of incidence, and distorted when it is parallel. 



Iodine /3 only shows the black cross very faintly with unpolarized 

 light when its axis is perpendicular to the plane of incidence. 



The distortion when the incident light is polarized is far greater than 

 that produced by the reflection of a ray of light, polarized at an angle of 

 45 degrees with the plane of incidence, from the surface of a piece of 

 glass, and quite comparable in amount ^^ith the effect produced Avhen 

 the light falls on a metallic surface. 



I selected from a large quantity of freshly sublimed iodine a few pieces 

 bearing a more regular crystalline form than the rest, and amongst these 

 there were two nearly triangular plates, about 10 mm. long and 8 mm. 

 broad. 



One of these was arranged on the stage of the polariscope, and the 

 light reflected from the surface observed in the way that has already 

 been described in the case of the films of iodine. 



The light was most completely polarized when the angle of incidence 

 was about 72° ; when the length of the crystal was perpendicular to the 

 principal section of the analyzer it appeared darkest, and when parallel 

 with it lightest. 



The surface-colour and the distortion of the black cross were as 

 clearly seen as with the film of iodine. 



After several unsuccessful attempts I succeeded in preparing some 

 well-defined crystals of iodine by carefully heating on a sand bath a 

 small quantity of that substance in a wide-mouthed stoppered bottle, 

 when some perfect rhomboidal plates of iodine about 1 mm. long 

 were deposited on the cool part of the bottle. When these were 

 examined with the polariscope, the principal section of the IVicol being in 

 the plane of incidence, they appeared brightest when their long axis was 

 parallel ^^^th, and darkest when it was perpendicular to, the plane of 

 incidence. 



Hence it appears that when the long axes of these crystals are parallel 

 with or perpendicular to the plane of incidence, part of the light reflected 

 from their surface is polarized in the plane of incidence, and part in a 

 plane at right angles to their long axes, and consequently that the long 



