Ch. II] 



DIAPHRAGMS AND LIGHTING 



47 



from all sides. If the bundles of light are made up of diverging or 

 of converging cones, then the axes of the cones should be coincident 

 with or parallel with and symmetrically 

 arranged around the optic axis of the 

 microscope (fig. 41-42). 



§ 87. Oblique light. — By this is 

 meant light which reaches the object 

 with its axial beam oblique to the optic 

 axis of the microscope. With oblique 

 light the object cannot be illuminated 

 equally from all sides, but largely from 

 one side, and consequently the light is 

 said to be unsymmetrical. 



If no condenser is used, oblique light 

 is obtained by turning the mirror so 

 that parallel rays strike the object ob- 

 liquely to the optic axis of the micro- 

 scope (fig. 35 c) or the axis of the 

 converging or diverging beam from the 

 concave mirror strikes the optic axis 

 obliquely (fig. 35). 



If a condenser is used, oblique illumi- 

 nation is produced by making the dia- 

 phragm opening eccentric, or most 

 simply by putting the finger or other 

 opaque body between the mirror and 

 the condenser to cut off part of the light 

 (fig. 46, 67). The end result in all cases 

 is that the object is lighted unsym- 

 metrically. 



§ 88. Use of a diaphragm. — A dia- 

 phragm is an opaque disc with an opening, and is placed somewhere 

 between the object and the source of light. 



At the present time an iris diaphragm is almost universally em- 

 ployed. It, like the iris of the eye, can be expanded or contracted, 

 and thus gives a large range of openings to meet different conditions. 



Fig. 35. High-power Im- 

 mersion Objective with Di- 

 rect and Oblique Trans- 

 mitted Light. 



Axis The principal optic axis. 



Mirror This reflects the 

 light up through the object. 



A B Direct light. 



C Oblique light. 



Stage The microscope stage 

 in section. 



The object. 



/ Immersion liquid between 

 the objective and object. 



F C The front lens of the 

 objective. 



M C The middle combina- 

 tion. 



B C The back combination. 



