187G.] 



Polarization of Light. 



57 



Experiments similar to those just described show that when a ray o£ 

 plane polarized light is incident upon such a surface of iodine, it is never 

 completely extinguished, as is the case with iodine /3 ; but the intensity 

 of the reflected light depends on the relative position of the plane of 

 polarization and the axis of the crystals, being least when they are 

 parallel. 



When the slips of glass between which the iodine has been melted are 

 carefully separated, the film usually remains attached to one of them in 

 a sufficiently perfect condition to be examined. At first it is extremely 

 brilliant, and shows exactly the same appearances as have already been 

 described as occurring with film of iodine under glass. The surface, 

 however, not only tarnishes rapidly, but even at a low temperature (10°) 

 the film quickly evaporates ; and consequently the uncovered films are 

 somewhat difficult to examine. 



They, however, permit some additional facts to be observed, which 

 either are seen with difficulty or not seen at all when the iodine is 

 covered with a plate of glass : and chief amongst these is the " surface- 

 colour " which iodine shows when light is incident upon it at a high 

 angle. 



When a film of either iodine a or /3 is placed on the stage of the polari- 

 scope with its axis parallel with the plane of incidence and the principal 

 section of the Mcol in the same plane, the surface of the iodine appears 

 bright and metallic when light is incident on it at an angle of about 60°. 

 As the angle of incidence increases, the colour of the reflected light 

 changes ; at about 70° the surface appears blue, and is still bright, but 

 has lost its metallic appearance to a considerable extent, and at about 

 72° the colour is most intense ; but as, in addition to the difficulties 

 which are inseparable from determiuations of this kind, the instrument 

 which I have used for these experiments does not allow of any very 

 accurate measurements being made ^^■ith it, the value of these angles can 

 only be regarded as approximate. 



On rotating the stand the amount of reflected light diminishes rapidly, 

 and the iodine appears dark or nearly so when the axes of the crystals 

 are perpendicular to the plane of incidence. 



On rotating the Nicol, the axes of the crystals of iodine remaining 

 parallel with the plane of incidence, the surface of the iodine becomes 

 bright and metallic, the maximum amount of light being transmitted 

 when the principal section of the Nicol is perpendicular to the plane of 

 incidence. 



When the incident light is polarized in the plane of incidence the 

 surface of the iodine appears brilliant and metallic in all positions, and 

 when seen through the analyzer the amount of light reflected by the 

 film alters as the former is rotated, but there is no trace of colour. 



When, however, the light is polarized perpendicularly to the plane of 

 incidence the reflection from the surface of the iodine is a coloured one 



