38 HOW TO USE THE MICROSCOPE 



under-stage collar until the outside surface of the 

 top lens is flush with the surface of the stage and 

 in touch with the under side of the object. The 

 lamp is placed 4 or 5 inches from the mirror, the 

 flam side of which is used, and light is directed 

 through the condenser. Then the object is focussed 

 with the objective in the usual way. With the iris 

 diaphragm nearly closed, the condenser is gradually 

 moved downwards in the under-stage collar until the 

 flame of the lamp is imaged in the field as a central 

 streak of light between dark margins. The dia- 



FiG. 1.3. — Abbe Condenser Mounted for Under-Stagb. 



phragm is finally adjusted to the most suitable aper- 

 ture. The edge of the flame is turned towards the 

 mirror. 



With the condenser thus used, the centre of the 

 field is critically and brightly illuminated, enabling 

 the worker to get the best possible results with 

 medium and high-power objectives, which invariably 

 give their best definition at one point of the field 

 only, and it is that point, in the centre, upon which 

 light is concentrated. If it is desired to light the 

 whole field, the flat of the flame may be turned to 

 the mirror, or rays from the flame can be parallelized 



