70 Mr. Baldock on the Polariscope presented by the President. 
ference effect. The colours of selenite films in the polariscope 
are thus produced. Selenite is a doubly refracting mineral 
which readily splits up into thin plates, a definite thickness of 
plates producing a definite pair of complementary colours. 
Some curious and amusing slides were exhibited, in which 
plates of selenite producing appropriate colours had been arranged 
to form pictures, with leaves and flowers. In one slide a miller 
was changed into a sweep by the turning of the analyser. 
