exhibited by certain Polarising Substances. 



435 



One of the strongest polarising substances I have come across is 

 the crystal epidote. The crystal is very small in size, and I could 

 not get with it complete absorption of one of the two rays. But it 

 exhibits very strong depolarisation effect, even with a thickness as 

 small as 0'7 cm. This is, undoubtedly, due to strong selective absorp- 

 tion in one direction. I cut a square from this crystal 0'7xO - 7 cm. 

 with a thickness of 0'4 cm. Using an E.M.F. of 14 volts the deflections 

 obtained (proportional to the two conductivities) were 105 and 20 

 divisions respectively. The conductivities in the two directions are, 

 therefore, in the ratio of 5'2 : 1. With an E.M.F. of 100 volts and a 

 -diminished sensibility of the galvanometer, the deflections were 205 

 and 40, the ratio of the conductivities being as 5'1 : 1. January 28, 

 1897.] 



It would thus appear that substances like nemalite which polarise by 

 double absorption, also exhibit double conductivity. It is probable 

 that, owing to this difference of conductivity in the two directions, 

 each thin layer unequally absorbs the incident electric vibrations ; 

 3,nd that by the cumulative effect of many such layers, the vibrations 

 which are perpendicular to the direction of maximum conductivity 

 are alone transmitted, the emergent beam being thus completely 

 polarised. 



[Owing to the great difficulty in obtaining suitable specimens, I 

 have not been able to make a more extended series of determina- 

 tions. The relation found, in the cases described above, between 

 double absorption and double conductivity is, however, suggestive. 



