134 MICROSCOPIC ILLUSTRATIONS. 



should be placed as near to it as it conveniently can 

 be, in order that the cone of rays reflected from the 

 mirror may spread over the whole surface of the silver 

 cup ; otherwise, the maximum illumination will not be 

 obtained. 



Having brought the object into focus, the silver cup 

 must be adjusted, without altering the adjustment of the 

 focus of the instrument. This is done by sliding the tube 

 belonging to the silver cup over the tube which is screwed 

 before the object-glass. In order to obtain the maximum 

 quantity of light, the cup must be placed so that the 

 object is a little within its focus. As the length of this 

 focus, however, varies with the distance at which the 

 candle, lamp, or other illuminating source is placed from 

 the cup (the rays being thus either parallel, diverging, 

 or converging,) the adjustment must be made after the 

 position of the illuminator is determined upon. Thus, 

 when the cup is used with a candle placed at 3 or 4 

 inches distance from it, the rays being divergent, the 

 focus will be elongated, in which case the two tubes 

 must be thrust in. Again, when we illumine by day- 

 light reflected from a plane mirror, the focus is shortened, 

 and the outer tube requires to be drawn out a little. 



In the old constructed microscopes, the concave spe- 

 culum, or silver cup, was attached to the stage of the 

 instrument, so that a fresh adjustment was always re- 

 quired for every change of object. By the above men- 

 tioned plan, as soon as the cup and light are adjusted, 



