20 OBJECTIVES FOR THE MICROSCOPE [Ch. I 



number, or the earlier in the alphabet the letter, the lower is the 

 power of the objective. This method is entirely arbitrary and does 



Fig. 21 a. Low Objective in Section. 



Axis The principal optic axis of the objective. 

 fl The front lens of the objective. 



be The back combination composed of a concave and a convex lens. 

 Stage The stage of the microscope in section. 



Mirror The mirror is above the stage in this case and reflects light down 

 upon the object. 



rl Reflected light from the object. 

 si The glass slide. 

 sp The specimen on the slide. 

 eg Cover-glass over the specimen. 



Fio. 21 B. High Power Objective in Section. 



Axis The principal optic axis of the objective. 



be Back combination of a double convex and a plano-concave lens. 



me Middle lens combination. 



fl Front lens of the objective. 



eg, sp, si The cover-glass, specimen, and slide. 



Stage The stage of the microscope in section. 



Mirror The mirror reflecting parallel rays up through the specimen. 



Fig. 21 C. High-power Objective of Four Combinations. 



I The front lens. 



2, 3, 4 The three combinations of lenses, the back combination (4) com- 

 posed of three lenses. 



not, like the one above, give direct information concerning the ob- 

 jective. 



