54 Sir Jolm Conroy on the [May 18, 



a more careful examination usually showed some traces of crystalline 

 structure in a direction parallel ^^ith the principal section of the Mcol. 



I shall refer to these two forms of iodine as iodine a and iodine /3, 

 calling" those portions of the film a which are of a da,rker colour and 

 polarize the light imperfectly, and the light-coloured strongly polarizing 

 parts /3. 



From the appearance of the film, when seen under the microscope, it 

 appears probable that this difference depends merely on the arrangement 

 01 the crystals, and that when they are regularly disposed in a single 

 layer the film is one of those which I hare called iodine (3 ; whilst iodine a 

 consists of several layers of thin crystals lying in various dh-ections, or 

 it may be due to different faces of the crystals of iodine being in contact 

 with the glass, and to the light passing through the crystals in a different 

 direction. 



The difference between the action of the crystalline film on the trans- 

 mitted hght is one of degree only; for I obtained two specimens of 

 iodine jo in which the film was of unequal thickness ; and in this case, 

 when the principal section of the INTicol was parallel with the long axes of 

 these crystals, the thick portion of them appeared opaque, but a con- 

 siderable amount of light was transmitted by the thinner portions of the 

 very same crystals. Moreover several specimens of iodine (3 w^hich 

 appeared perfectly opaque in certain positions of the Nicol when seen by 

 ordinary dayhght, were of a deep red colour when examined by direct 

 sunlight ; and I have recently succeeded in preparing several films of 

 iodine /3 so thin that they w^ere only opaque when seen through a Mcol, 

 whose principal section was parallel ^ith the length of the crystals, with 

 very weak light; by ordinary daylight they appeared of a deep red 

 colour under these circumstances. 



Iodine a. — Films of iodme a between Wo sHps of glass were laid on 

 the stage of the polariscope, and the light they transmitted examined 

 with the analyzing Xicol ; on turning either the stage or the Mcol, the 

 colours of the film varied, according to their thickness and the relative 

 positions of the film of iodine and the Mcol, from a kind of brownish 

 yellow to a deep red, the colours being similar to those of solutions of 

 iodine in alcohol of various strengths. 



When the incident hght was polarized, and the film of iodine placed 

 so as to transmit the minimum amount of light, or at right angles to 

 tills position, the field was dark when the Mcols were crossed and light 

 when they were parallel. When, however, the film of iodine was in an 

 intermediate positiou, the field was no longer dark in any position of the 

 analyzer, the colour and intensity of the light varying slightly as it was 

 turned. 



Iodine /3. — A film of iodine /3 of a yellowish browii colour was ex- 

 amined by transmitted hght ; the field appeared perfectly dark in two 

 positions of the film and Mcol. With polarized light, when the film vras 



