104 MICROSCOPIC ILLUSTRATIONS. 



lateral motion. On Dr. Goring's principle, as described 

 in the next chapter, a corresponding effect, is produced 

 by moving the optical portion of the instrument; a 

 system which doubtless has its advantages, when either 

 the aquatic larvae of insects, or other large living objects, 

 are examined ; as also when any sizeable bodies are 

 being dissected under the regular microscopes employed 

 for that purpose. The moveable stage, however, for 

 small objects, and for general purposes, is usually pre- 

 ferred. 



The ordinary method of adjusting a moveable or tra- 

 versing stage is by means of a couple of screws working 

 at right angles to each other ; by turning one or both of 

 which, a plate affixed to them, and carrying with it the 

 slider-holder containing the object, is moved in any di- 

 rection. Sometimes a screw and a rack and pinion are 

 substituted, by means of which all the movements are 

 effected on one side; at others, levers are made to 

 answer the purpose. But by far the most elegant in- 

 vention is that of the late Mr. Tyrrel, the celebrated 

 mechanical engraver, which, together with some im- 

 provements 1 have made upon it, I propose to describe, 

 as being the one most likely to be adopted generally, and 

 preferred to all others. Its advantage consists in that 

 with one hand and without removing it, you can give to 

 the object a lateral movement in all directions. This is 

 done by attaching a screw to the lower fixed plate of the 

 stage, within whose axis passes the axis of a smttll 

 pinion, the outer extremity of each terminating in a 



