1892.] 



MICllOSCOPIOAL JOURNAL. 



287 



the lacquer with a brush, as it leaves unsightly ridges. Nearly all 

 retort stands have light bases ; therefore remember that when the 

 '• centre of gravity falls outside the base " you are very likely to 

 "•' hear something drop." 



a. Frame of ground glass. 

 6. Card reflector. 



EXPLANATION OF PLATE. 



c. Wire support for reflector. 



d. Clamp. 



By the way, a ^-inch thin glass circle, if coated with the blue 

 lacquer and stuck on the lower end of the cylinder holding the 

 diaphragms (in a continental stand) with a little paraffin, makes 

 a good ^ moderator" for night work. 



MICROSCOPICAL MANIPULATION. 



Practical Points in Handling Objectives to Obtain 

 Best Definition. — If you want to compensate for thinner cover- 

 glass, set the systems of your objective further apart; or the same 

 purpose may be effected by lengthening the tube of your micro- 

 scope. If, on the contrary, you want to correct for a thicker 

 cover-glass, set the systems closer, or make your tube shorter. 



I. Thicker cover, longer tube, and opening systems tend to 

 over-correction. 



II. Closing systems, thinner cover, and shorter tube tend to 

 under-correction. 



For the recognition of under or over correction by the appear- 

 ance of the object, the writer has found the method of E. Gund- 

 lach to be of great practical value ; and he would urge careful 

 practical study of these appearances as affording a guide to the 

 kind of correction needed, whether '' under" or " over." 



To illustrate the practical use of the above, suppose, for ex- 

 ample, a condition of under-correction of " general spherical ab- 



