Sir Jolm Conroy on the 



LMay 18, 



position of the Nicol, the fihn of iodine was rotated horizontally, the 

 Nicol remaining at rest, the same changes in brilliancy occnrred . 



Hemo^dng the upper glass made no difference, except that the surface 

 of the film of iodine tarnished rapidly, and then the amount of light re- 

 flected by it became considerably less. 



The light incident upon the surface of the iodine was either ordinary 

 diffused daylight or the light of a parafhne-lamp ; and in neither case 

 did it show more than the merest trace of polarization, and generally not 

 even that, when examined by means of a double-image prism and a plate 

 of selenite. 



It therefore appears that the light reflected from the surface of a layer 

 of iodine is polarized, and that the position of the plane of polarization 

 is not, of necessity, either parallel or perpendicular to the plane of in- 

 cidence, but bears a definite relation to some direction within the crystals 

 composiug the film. 



I also found that when these films were sufficiently thin to be trans- 

 parent the light they transmitted was polarized, and that the plane of 

 polarization of the transmitted light was perpendicular to the plane of 

 the light which was polarized by reflection from the same portion of the 

 film. 



After making these observations I ascertained that W. Haidinger had 

 announced, upwards of twenty years ago (Pogg. Ann. clxxi. p. 321, 1847), 

 that the surface-colours which certain substances show by reflected light, 

 in the case of some of the platino-cyanides, namely, those of potassium, 

 barium, and magnesium, consist partially of light polarized in a plane 

 which bears a definite relation to the axis of the crystal ; and in a sub- 

 sequent paper (Sitzungsberichte der kaiserlichen Akademie der "Wissen- 

 schaften, viii. p. 97, 1852) he states that certain other substances have 

 the same property. He mentions iodine in this latter paper as showing 

 these " surface-colours," but does not appear to have noticed that the 

 plane of polarization of the light reflected from its surface bore any 

 relation to some fixed direction \^dthin the substance. 



I arranged a form of polariscope ]}y means of which these observations 

 could be repeated Vs^th. a greater degree of accuracy. The instrument 

 used consisted of a divided brass circle fixed verticall}^ to a firm support ; 

 a Nicol furnished with a graduated circle was carried by an arm moving 

 round the centre of the circle, and the slip of glass with the layer of 

 iodine rested horizontally on a stage at the top of a tube, the height of 

 which could be adjusted so that the surface of the iodine was level with 

 the centre of the graduated brass circle. Both the stage and the tube 

 revolved horizontally, and could be rotated independently of each other ; 

 and the latter had an index moviug over a divided circle attached to it, 

 and a diaphragm with an opening about G mm. wide fixed in it. By 

 altering the position of the arm mo^dng ovei' the vertical circle the light 

 reflected from the surface of the iodine at different angles could be ex- 



