Royal Microscopical Society. 199 



sKding under two staples c c. At (Z is a spring for forcing the 

 glass against the set-screw e. On the top surface of the glass 

 there is a strip of platinum foil, "001 thick, cemented on with 

 Canada balsam. Fixed underneath the opposing staple there is a 

 similar piece of foil. By turning up the set-screw to a stop the 

 straight edges of the foils are brought in contact, and opened by 

 reversing the screw. 



This instrument is placed on the stage of a microscope, with 

 the body horizontal and set on a wooden turn-table about ten inches 

 in diameter, having its edge divided into degrees. The object-glass 

 to be measured is focussed on the glass surface, and the sht ad- 

 justed so that its two edges just appear in the field of view, and by 

 the rotation of the turn-table the degrees of aperture are read off. 

 If the microscope has a thick stage this may cut off the rays for 

 large angles. In this case a cell like an inverted Hve-box may be 

 made to pass through to the under-side, to which the adjustable slit 

 is fixed, and if the object-glass will not reach it must be extended 

 by an adapter. 



