152 Transactions of the Society, 



been rotated into the position in which the colour of the selenite 

 just disappears. A selenite slide showing various tints does very 

 well for this purpose, and I am using home-made ones, split from 

 some crystals which I found in some Kimmeridge clay dug from a 

 well. 



In claiming that these simple polarizing devices give much 

 better results than one might expect it is not meant that they are 

 free from defects. Apart, however, from their usefulness in the 

 rough-and-ready examination of objects previous to final mounting 

 there is a great interest to be derived by experimenting with the 

 apparatus here described. 



