Ch. II] EXPERIMENTS WITH A SUBSTAGE CONDENSER 



63 



filled if a proper illuminator or condenser is used. This is because 

 light rays go off from a light source in the form of a sphere, and 

 even a point source would give light which would fill any aperture. 

 The aperture in a given case depends on the diaphragm used with the 

 condenser, and the size of the diaphragm must vary directly as the 

 aperture of the objective. That is, it is just the 

 reverse of the rule for diaphragms where no con- 

 denser is used (§ 89) ; for there the diaphragm is 

 made large for low powers, and consequently low 

 apertures, while with the condenser the diaphragm 

 is made sjnall for low and large for high powers, 

 as the aperture is greater in the high powers of 

 a given series of objectives. It is very instructive 

 to demonstrate this by using a 16 mm. objective 

 and opening the diaphragm of the condenser till 

 the back lens is just filled with light. Then if one 

 uses a 4 mm. objective it will be seen that the 

 back lens of the higher objective is only partly 

 filled with light and to fill it the diaphragm must 

 be much more widely opened. 



§ 111. Mirror and light for the condenser. — 

 It is best to use light with parallel rays. The 

 rays of daylight are practically parallel; it is best, 

 therefore, to employ the plane mirror for all but 

 the lowest powers. If low powers are used the whole field is not 

 illuminated with the plane mirror when the condenser is close to the 

 object; furthermore, the image of the window frame, objects out- 

 side the building, as trees, etc., would appear with unpleasant dis- 

 tinctness in the field of the microscope. To overcome these defects 

 one can lower the condenser and thus light the object with a diverg- 

 ing cone of light, or use the concave mirror and attain the same end 

 when the condenser is close to the object (fig. 42). 



§ 112. Lighting the entire field with a condenser. — With the 

 condenser there are two conditions that must be fulfilled; the proper 

 aperture must be used, and the whole field must be lighted. As seen 

 in § no the diaphragm of the condenser regulates the aperture of the 



Fig. 41. Substage 

 Condenser of 

 Abbe with Paral- 

 lel Light. 



Axis The prin- 

 cipal optic axis of 

 the condenser and 

 objective (Ob). 



The iris dia- 

 phragm (D) is. be- 

 low the condenser. 



