1876.] 



Polarization of Light. 



55 



placed at right angles to the position in which it transmitted no light, 

 the field was dark when the Mcols were crossed and light when they 

 were parallel. When, however, the iodine film was placed in an inter- 

 mediate position, the field was no longer dark ^^'hen the Xicols were 

 crossed, though it was so in two positions o£ the analyzer 180° apart. 



In order to simplify the description of the experunents, I shall speak 

 of the direction in the film which, when placed parallel with the principal 

 section of the Mcol, caused the field to appear dark, or in the case of 

 iodine a to be least bright, as the axis of the crystal. 



Very thin films of iodine /3, as I have mentioned before, are not 

 opaque when the principal section of the Mcol is parallel with the length 

 of the crystals, and, when examined with the polariscope, appear of a 

 deep red colour, when, under similar circumsta^nces, a thicker film would 

 transmit no light at all. 



Hence it would appear that iodine belongs to the class of double re- 

 fracting substances in which the coefiicient of absorption differs accord- 

 ing to the direction in which the light passes through the crystal, and, 

 further, that the ray whose plane of polarization is perpendicular to the 

 axis of the crystal is most energetically absorbed. 



This is the case with both forms of the crystalline layer of iodine ; but 

 the two rays are much more unequally absorbed by iodine /3 than by 

 iodine a — so much so that whilst the latter only appears absolutely opaque 

 when the principal section of the Mcol and the axis of the crystal are 

 parallel, when the film is so thick that but little light can pass through 

 under any circumstances, the former absorb the one ray so ener- 

 getically that a layer which appears light yellow when the Mcol is in 

 one position is absolutely opaque when it is turned through an angle 

 of 90°. 



When a thin film of iodine (j is seen through a Mcol whose principal 

 section is so placed that the minimum amount of light is transmitted, 

 the light usually appears of the same colour and brightness as that which 

 has passed through the adjacent portions of the film consisting of 

 iodine a; and it is impossible to see where one form of the film ends 

 and the other begins. From this it would appear as if both forms of 

 the crystalline layer absorbed light polarized in a plane perpendicular to 

 the axis' of the crystals with equal intensity, but that they differ 

 greatly in their absorptive powers for light polarized in a plane at right 

 angles to this. 



I have shown, in the preceding paper, that solutions of iodine in alcohol, 

 when seen by transmitted light, vary in colour, from a pale yellow to a 

 deep red, according to the strength of the solution and the thickness of 

 the layer through which the light has to pass. In a similar manner, in 

 proportion as the thickness of the films of iodine increases, the light be- 

 comes more and more red ; and four films of iodine /3, which when seen 

 separately were of a pale yellow, appeared of a deep red when super- 



