1876.] 



Polarization of Light, 



53 



amined ; and by reflecting light along the axis o£ the tube, by a mirror 

 placed below it, and clampiDg the arm in such a position that the axis o£ 

 the tube and ]N'icol were in the same straight line, the polarization of the 

 light transmitted by the iodine could also be observed. A second or 

 polarizing Nicol was so arranged that it could either be brought below 

 the tube or placed between the surface of the iodine and the source of 

 light, so that the behaviour of the film, ^^ hen the incident light was pola- 

 rized, could be studied. 



On repeating the before-mentioned observations with reflected light I 

 found that occasionally portions of the film of iodine appeared quite 

 black in certain positions of the film and Mcol, and that these same 

 portions, when examined by transmitted light, did not merely alter in 

 colour as the film or Nicol w^as rotated, as the remainder did, but in 

 certain positions transmitted no light at all — or, in other words, that they 

 behaved in a similar manner to what a plate of tourmaline would have 

 done ; and when seen by ordinary light and the naked eye, although they 

 appeared to be of the same thiclmess as the remainder of the film, by 

 which they were wholly or pcirtially surrounded, they wqvq of a much 

 paler colour ; usually, moreover, they reflected rather less light than the 

 rest. 



The change in the appearance of these portions of the film when seen 

 through a Nicol was very striking in the case of some of the larger ones, 

 as in certain positions they appeared perfectly transparesit and of a pale 

 yellow colour, and objects situated behind them could be clearly seen ; 

 but on turning either the film or the Mcol they became perfectly opaque, 

 and resembled highly polished metallic surfaces. 



On examining one of these films of iodine with a microscope with a 

 I object-glass I found that those portions of the film which polarized 

 light most strongly differed considerably in appearance from the re- 

 mainder, and that they appeared to consist of long crystals about 

 •003 inch wide adhering together side by side, whilst the rest of the film 

 seemed to consist of thin plates of iodine overlaying one another, these, 

 also, being long in proportion to their width. 



A Mcol prism was placed over the eyepiece, and I then found that 

 when the principal section of the Xicol was perpendicular to the long 

 axes of the crystals, the maximum amount of light was transmitted, 

 and when the principal section was parallel A^ith the long axes of the 

 crystals, they either appeared perfectly opaque or transmitted the 

 minimum amount of light, according as the crystals in the field of view 

 belonged to one or other class. 



Other specimens, of iodine showed this crystalline arrangement with 

 different degrees of distinctness ; but in all, or nearly all, some trace of 

 it could be seen. In some cases, however, the minimum amount of light 

 was transmitted ^^ hen the principal section of the Nicol was not parallel 

 with what appeared at first sight to be the long axes of [h'3 ciystals; but 



