ACCESSORY INSTRUMENTS. 



115 



attached to the stage, b a portion of large tube, having affixed 

 to it a ring of brass, into which is soldered a smaller tube 

 carrying the pinion with a milled head, f; within this tube 

 a still smaller one, d, with a screw at the top to carry the 

 illuminator, g, and a diaphragm at the bottom, to cut off all ex- 

 traneous light, is moved up and down by a rack, in which works 

 the pinion, e. The vertical adjustment of this instrument is made 

 by the small screw attached to the plate, a a, whilst all the other 

 movements are effected by turning three or more screws in 

 the ring of brass, by which the inner tube carrying the Ulumi- 

 nator, can be moved in various directions, so as to bring its 

 axis to coincide with that of the object-glass. Two of these 

 screws are seen at c and d. This plan was first suggested by 

 Mr. Koss, and is adapted to all his instruments in which the 

 arm carrying the compound body is a fixture. The several 

 parts of the illuminator, g, unscrew, so that they may be used 

 either combined or separate. 



The achromatic condenser supphed with the largest micro- 

 scopes of Messrs. Smith and Beck, is represented by figs. 

 60 and 61 ; and for the better exhibition of its several parts, 



Fig. 60. 



