6 4 



EXPERIMENTS WITH A SUBSTAGE CONDENSER [Cn. II 



illuminating cone, but has nothing to do with lighting a large or a 

 small field. The size of field that is lighted by means of a condenser 

 can be modified in two ways: 



(i) Suppose that the image of the source of light is focused on the 

 object, the size of that image will determine the size of field which is 

 illuminated in a given case. If the illuminated field is not so large 

 as the objective field, then the source of light is 

 too small, or too far away. In that case use a 

 larger source or bring the source closer to the 

 microscope. 



(2) By lowering the condenser or using the con- 

 cave mirror a much larger object can be fully 

 lighted, as it is in a diverging cone of light above 

 the focal point of the condenser where the light 

 is spread over a greater area (fig. 42). 



For quite low objectives, 35 to 60 mm. focus, it 

 is better to remove the condenser and use the 

 mirror only. The whole field can be illuminated 

 easily and sufficiently in this way. 



113. Homogeneous immersion condenser 

 (Ch. IX). For numerical apertures higher than 

 i. oo it is necessary to connect the under side of 

 the microscopic slide with the top of the con- 

 denser with homogeneous immersion fluid. As 

 the objectives used for high apertures are also 

 homogeneous immersion, there is no change in the direction of the 

 light from the condenser until it reaches the objective, excepting dif- 

 fraction effects, and the reflecting or refracting action of the speci- 

 men in the path of the rays. The special need of the homogeneous 

 connection between the slide and the condenser comes from the laws 

 of refraction. If there were not a homogeneous connection the most 

 oblique rays, that is, those giving a numerical aperture above i.oo 

 with a solid cone of light, and the most oblique rays with a hollow 

 cone of light, would meet the terminal surface of the condenser at an 

 angle greater than the critical angle, 41 + , and would therefore be 

 totally reflected as from the surface of a mirror, and the advantage 



FIG. 42. SUB- 

 STAGE CONDENSER 

 WITH CONVERGING 

 LIGHT. 



A xis The prin- 

 cipal optic axis of 

 the condenser and 

 the objective (Ob). 

 The iris diaphragm 

 (D) is below the con- 

 denser. 



