796 Botanical Microtechnique 



out of focus and may not stop until the slide is smashed. As an 

 alternati\e arrangement the high-power objective may be parfocalized 

 so that, when it is swung into position, the image is visible and a 

 slight downward movement brings it into sharper focus. An accidental 

 movement in the wrong direction, upward, will then do no harm. 

 Students should be told firmly that there is no excuse for ttuning a 

 fine-adjustment knob more than a half revolution in either direction. 

 On the best modern microscopes very little pressure is exerted on the 

 slide when the bod\ tube is lowered upon it with the fine adjustment. 



The condenser and illuminant are introduced again at this point. 

 Assume that a grainless disk of a lamp serves as the immediate source 

 of light. Adjust the height of the condenser until the surface of the 

 disk is in view simultaneously with the focused specimen. The object 

 is then within a disk of light of uniform linninosity. Ob\ iously, the 

 condenser does not project a point of light, but a disk of light in the 

 plane of the specimen. Although a groinid-glass source approaches 

 the requirements for correct illumination, the condenser must be 

 lowered to move the granularity out of focus. An opal glass disk 

 permits a closer approach to correct illumination. 



For general class use, the most practical light source is a lamp with 

 a grainless or nearly grainless diffusing disk. It is preferable to have 

 the lamp fastened to the table in constant relation to the position of 

 the microscope. Under such conditions, the condenser, especially the 

 low N.A. condenser described on jiagc 192, can be mounted in sinijile 

 ring mounts that are not adjustable by the students. 



Ihc position of the microscope in use depends to some extent on 

 the height of the available table and chair in relation to the physical 



lu.. 1{)..'! — Hiiigc siDj) lor class- 

 room microscope. 



l)iiil(l ol the user. Hard and last iiilcs ol posliu'e are lidicidoiis iu a 

 classroom Iia\ing tables aud (liaiis ol lixed. uuilonn height, and 

 students of di\cise l)iiil(l. A \(i\ short person shouUI (ertaiuh lili the 



