120 PHOTOMICROGRAPHY 



to focus an image of the radiant across the centre of the 

 object, which is simultaneously in focus, and made sure 

 by examination of the back lens of the objective that 

 three-quarters or more of its N.A. is in use, the con- 

 ditions laid down by Mr. Nelson as governing the pro- 

 duction of the critical image have been attained. But the 

 appearance of the object itself, with the image of the 

 lamp-flame forming a bright streak across it, is not 

 suitable for photomicrography. The part illuminated is 

 shown to the best advantage possible, but some means 

 must be taken to equalise the light over the whole field 

 before a useful photomicrograph can be taken, and the 

 means by which this is done, without depreciating the 

 image, are most important. 



7. Effect on the Field of View of Moving the Condenser. 

 As the condenser is racked above or below its focus 

 notice that the illumination over the field is reduced, 

 and an effect similar to closing down the iris is observable 

 on the back lens of the objective. But it is often possible 

 by a very small movement from exact focus to equalise 

 the light over the field, and do away with the image of the 

 radiant, without any apparent restriction of the N.A. of 

 the objective. 



8. The Relation of the Focal Length of the Condenser 

 to the Power of the Objective. (a) Still keeping the f" 

 objective in position, remove the front lens of the con- 

 denser, or replace the original condenser by one of longer 

 focal length. Note that the remaining combination must 

 be racked down to form a sharp image of the radiant 

 across the object, and also that the image is larger than 

 that formed by the complete condenser. It may cover the 

 entire object, or fill the field of the objective. 



(b) Focus the complete condenser again, and replace 

 the low objective by a higher power, a J" for instance. 

 The entire field is now probably covered evenly by the 

 image of the light. 



