38 l- Royal Society : — Mr. W. Crookes on a New Arrangement 



*-ineh focus or longer; and an object as small as a single corpuscleof 

 blood can be examined and its spectrum observed. 



The two additions to tbe microscope consist of the substage with 

 slit &c, and the prisms in their box. The substage is of the ordi- 

 nary construction, with screw adjustment for centring, and rackwork 

 for bringing it nearer to or withdrawing it from the stage. Its 

 general appearance is shown in tig. 1, which represents it in position. 

 A B is a plate of brass, sliding in grooves attached to the lower part 



of the substage; it carries an adjustable slit, C, a circular aperture, 

 1), ti inch in diameter, and an aperture, O, g inch square. A spring 

 ivip enables either the slit or one of the a\ ertures to be brought into 

 the centre of the Held without moving the eve from the eyepiece. 

 Screw adjustments enable tae slit to be widened or narrowed at will, 

 and also varied in length. At the upper part of the substage is a 



