W. HISLOP ON A SIMPLE FORM OF SELENITE STAGE. 13 



lower side. The central portion of the slide is turned out with a flange, 

 so that a brass ring may be made to revolve in it easily. The film of 

 selenite, and the film of mica are cemented into this ring in the pro- 

 per relative position to give the desired effect, and eight notches 

 are cut in the edge of the disc. When the mounted object is 

 placed on this brass slide, the edge of the disc projects beyond the 

 slip, and it is easy to turn it through a portion of a revolution 

 without disturbing the object, by using a hooked wire or even the 

 point of a penknife. 



If now the colour given during the rotation of the analyzer be 

 blue and yellow, by simply turning the disc holding the compound 

 film through one-eighth of a revolution these colours will give place 

 to their complementary ones, namely, — red and green ; and they 

 will be given as brilliantly as they can be obtained by more compli- 

 cated arrangements. Intermediate positions give other tints. 



Further, this contrivance enables us to employ that most effective 

 element, namely, the rotation of the depolarizing film to suit the 

 various planes of double refraction in the object under examination, 

 and it does this in an easy and inexpensive manner. 



