166 



PRACTICAL PHOTO-MICROGRAPHY 



therefore its image more than fills the field, a good plan is to 

 place a metal plate with a small hole in it as near as possible 

 to the illuminant, so that only the centre of the latter is 

 exposed to view. Then centre the image of the aperture in the 

 plate this, in fact, being treated as the source of light. This 

 will only be necessary when a large-flame oil lamp is used, as 

 no other illuminant is of sufficiently large area to more than 

 fill the field when using low- or medium-power objectives. 



A[ 



JA 



Fio. 58. Diagram of Path of Rays in Sub-stage Condenser. 



All these adjustments are best effected with a low-power 

 ocular. The operations are similar whether the whole of the 

 sub-stage condenser is used as in the case of high-power 

 objectives or only the back combination. The advantage 

 of using a well-achromatised condenser and one having a large 

 aplanatic cone will, during these operations, become evident. 

 The image of the radiant should appear bright and well defined 

 in the field, and that portion of the field that is not covered by 

 the radiant should appear relatively dark. 



Fig. 58 shows diagrammatically the condition that should 



