46 THE ELEMENTS OF VEGETABLE HISTOLOGY 



position and locating the different working parts, 

 clean the lenses with cloth (soft silk or linen) or lens 

 paper, turn the iris diaphragm to full opening and 

 place low power objective (No. 3 or 16 mm.) in posi- 

 tion, 5 mm. above the stage. Turn the mirror so 

 that it half faces the north windows and, while looking 

 into the ocular, move the mirror slightly in different 

 directions until you obtain a clear white field. After 

 securing proper illumination, do not move the micro- 

 scope or the mirror. It is better to use the plane 



A. With plane mirror. 



B. "With concave mirror. 



Fig. 23. — Illumination of Object by Mirror, without Condenser. 



(Bausch & Lomb.) 



mirror unless the light is poor. Direct sunlight should 

 not be used for illumination, as it is so strong that 

 it will injure the eyes of the worker. 



Focusing with Low Powers. — After having obtained 

 proper illumination, place the slide, cover-slip side 

 upward, upon the stage so that the object is in the 

 center of the condensing lens. Lower the tube with 

 coarse adjustment so that the objective is about 3 mm. 

 above the specimen. Never focus downward with the 

 coarse adjustment unless you are watching the distance 

 between the objective and the specimen. Slowly focus 



