28 VEGETABLE HISTOLOGY. 



power is increased the working distance is decreased. It is 

 often advantageous to gain working distance at the expense of 

 magnification, as the manipulation of objects on the section 

 slide is made easier. 



2. PENETRATING POWER OR FOCAL DEPTH. This is the ver- 

 tical range through which the parts of an object not precisely 

 in the focal plane may be seen with sufficient distinctness to 

 enable their relations with what lies exactly in that plane to be 

 clearly traced out. It is larger the smaller the magnifying 

 power and numerical aperture are, and vice versa. Of two 

 objectives having the same power, but different working dis- 

 tances, that one will have the more focal depth whose working 

 distance is the greater. It is often desirable to see for a con- 

 siderable distance into an object. In such cases low power 

 must be used. 



3. FLATNESS OP FIELD. All parts of the image must be in 

 focus at the same time. 



4. DEFINING POWER. The power to form an image in the 

 highest degree sharply defined, and free from color. This 

 quality is governed by the objectives only and depends on accu- 

 rate centering of the lenses and completenss of correction for 

 spherical and chromatic aberrations. Want of defining power 

 is indicated by blurring of clearly-marked lines or edges and by 

 general fog. 



5. RESOLVING POWER. By which very minute and closely 

 approximated markings, whether lines, striae, dots or apertures 

 can be separately discerned. This power varies directly as the 

 aperture of the objective. High powers have the greatest re- 

 solving power. 



CARE OF THE MICROSCOPE. The stand should never be wetted 

 with such substances as alcohol, soap, etc., which dissolve 

 lacquer. If it be necessary to clean the stand, moisten with 

 water and dry with an old linen rag, rubbing with the grain 

 of the brass. Never examine objects lying in acids or alkalies 

 or other chemicals without putting on a "cover" glass. If 

 liquid happens to get on the objective, rinse off at once with 

 water and dry with an old linen rag or Japanese filter paper. 



Be careful not to force the lens down on the cover glass. 

 Exercise great care in putting objectives and eye-pieces on or 

 off, lest they be dropped and injured. 



DIRECTIONS FOR USING THE MICROSCOPE. 



1. The instrument should be placed directly in front of the 

 observer, with the pillar facing backward. Wipe the mirror 

 with a soft rag and turn it so that a beam of light is thrown up 

 through the diaphragm. All ivork should be begun with the 



