THE MICROSCOPE. 9 



animation. In the centre is a circular opening, for 

 the passage of the light reflected upward by the 

 mirror, H. There is also a sliding ledge, // against 

 this the glass slide, on which the object is mounted, 

 rests, when the Microscope is inclined from the 

 perpendicular. 



In a stage of this kind the various parts of an 

 object can only be brought under the eye by 

 shifting the slide with the fingers. But in more 

 expensive instruments the stage is usually con- 

 structed of one or two sliding plates, to which 

 motion is given by rackwork and pinion ; the 

 whole being brought under the hand of the operator 

 by two milled heads, a mechanical arrangement 

 which enables him to move with ease and certainty 

 the object he may wish to investigate. 



Underneath the stage 

 is the diaphragm, K, a 

 contrivance for limiting 

 the amount of light 

 supplied by the mirror, 

 H. It consists of a 

 thin, circular, flat plate 

 of metal, turning on a 

 pivot, and perforated 

 with three or four cir- 

 cular holes of varying 

 diameter (fig. ^ 4), the Dia p hra gm. 



largest only being equal 



to the aperture in the stage. By turning the plate 

 round, a succession of smaller openings is brought 

 into the centre of the stage, and in one position of 

 the diaphragm the light is totally excluded. By 

 this small but useful contrivance the Microscopist 

 can adjust the illumination of the mirror to suit 

 the character of the object he may be investiga- 

 ting. In some Microscopes the diaphragm is a fix- 



