THE MICROSCOPE. 35 



inches focus, A should be one inch, and their distance apart 

 two inches. 



Sometimes, when a very flat field of view is required, as in 

 the use of a micrometer eye-piece, the convex sides of the 

 lenses face each other. It is recommended that for this kind 

 of eye-piece the lenses should be nearly of the same focal 

 length, and at a distance equal to two-thirds the focal length of 

 either. 



A good compound microscope should be furnished with 

 many mechanical conveniences, in addition to the optical part 

 just described. It should be capable of being steady in any 

 position from vertical to horizontal have coarse and fine ad- 

 justments for focus have a large and firm stage, with ledge, 

 clips, &c. ; and with traversing motions, so as to follow an 

 object quickly, or readily bring it into the field of view, and 

 should have a concave and plane mirror, of two inches diame- 

 ter, with a universal joint, and capable of being brought nearer 

 or farther from the stage, as likewise of reflecting a side-light. 



