782 



RECORD OF CURRENT RESEARCHES RELATING TO 



It was made a full circle, in order to use it for another purpose also ; 

 but a semicircle of the same size would answer equally well. Into a 

 hole in the centre a pin is fitted, on which swings the radial arm B, 

 which carries on one side of the centre of rotation the body of a Micro- 

 scope E, while its extremity is provided with a vernier clamp D and 



Fig. 17. 



tangent screw C. On the other side of the central pin the radial arm 

 carries a table, on which is mounted a semicircle of crown glass F of 

 about two inches radius and half an inch thick. This is so mounted 

 that the edge, which corresponds to the diameter of the semicircle, is 

 directly over the centre of rotation, and the Microscope objective can 

 be focussed exactly upon the centre of the semicircle. At this spot a 

 thin glass cover, silvered except at a central circular hole (or ver- 

 tical slit) about 2^^ of an inch in diameter, is cemented with Canada 

 balsam, the central hole (or slit) being fitted precisely over the centre 

 of the semicircle. A suitable achromatic convex lens (a 4-inch 

 objective answers very well) is screwed at the end of the draw-tube of 

 the Microscope body, and serves to convert it into a telescope, pre- 

 cisely as in the apparatus of Abbe.* 



The particular method for which the apparatus was constructed 

 and by which exact measurements can be made, is as follows. Using 



* This apparatus can be used precisely like Ijis, if the semicircle is engraved 

 to degrees, and two shutters or indices provided. 



