228 W. HISLOP ON A NEW ANALYZING SELENITE STAGE. 



Some years ago the late Mr. Darker constructed an apparatus, 

 which he called a selenite stage. This consisted of a hollow plate 

 of brass, in the centre of which was a cell capable of being re- 

 volved around its centre by means of a tangent screw. Into the cell three 

 selenites, inserted in thin brass rings, were dropped. On the edge 

 of each of the rings the axis of polarization of the contained selen- 

 ite was marked, so that the axes of each might be made to coincide, 

 or they might be placed at any relative angle. The plate so pre- 

 pared was then placed on the stage, and the selenites revolved 

 together while the object was over them. The important element 

 of rotation was here provided for ; but the amount of interference, 

 and therefore the actual tint could only be changed by removing 

 the object, taking out the selenites, and replacing them at different 

 relative angles. Subsequently the selenites have been mounted in 

 separate moveable rings placed under the stage. This method 

 secures separate rotation, but is not convenient for rotation together. 

 The position of the selenites cannot be conveniently noted, and they 

 are too far from the object. 



Having been for some time engaged in observing the character- 

 istics of microscopic crystals, I found it desirable to provide some 

 means whereby, without separating the elements of the polarizing 

 combination to any appreciable extent, all the adjustments for inter- 

 ference and tint should be effected without disturbing the object in 

 the slightest degree. I wished, in fact, to place my object on the 

 stage, and examine it there under every possible variation of the 

 polarized beam. In conversation with Dr. Dempsey I found that he 

 also had experienced the need of an apparatus for a similar purpose, 

 in examining microscopic preparations of different classes, and I 

 was urged by him to contrive such an instrument. 



The result was the apparatus of which you have a diagram 

 before you, and which was first exhibited to the members of the 

 Club here, and subsequently at the soiree of the Old Change Micro- 

 scopical Society, Feb. 15th; at our own soiree on March 12th; and 

 at other meetings, by Dr. Dempsey and myself. See Figs. 2 and 3 

 (on next page). 



I have endeavoured, by this instrument, to give the selenites 

 every possible facility of adjustment, not in one direction only, but 

 backwards and forwards, together and separately. It consists of two 

 plates of brass, forming a cell or shallow box. In the centre of these 

 plates are two circular orifices, in which, and between the plates, 



