56 PHOTOMICROGRAPHY 



a sharp image if a coloured light replaces the white 

 without alteration of the fine adjustment. That is, 

 an object can be focussed with white light, a colour 

 screen then introduced, and a photomicrograph taken 

 without further adjustment, which is, as a rule, not 

 possible with achromats. 



7. They are also very desirable for visual use, as they 

 do not put such a strain on the accommodation of the 

 eye, and can be used for longer periods without fatigue. 



Apochromats and Achromats compared for Photography. 

 From what has been said, it will be gathered that 

 the objectives par excellence for photomicrography are 

 apochromats, and they should always be chosen if 

 their cost does not prevent it. But the question naturally 

 arises as to whether much is lost by substituting the 

 less expensive achromats for them ; and the results 

 given by the modern achromatic objectives, and par- 

 ticularly those containing fluorite or described as semi- 

 apochromats, are so good, when used with suitable 

 screens (Chapter IX), that the majority of workers 

 need feel no reluctance in selecting them. At the same 

 time, having once become accustomed to apochromats, 

 either for visual or photographic purposes, one would 

 very reluctantly go back to achromats. High-power 

 achromats have usually one distinct advantage over 

 corresponding apochromats, namely, a flatter field. 



Testing Objectives. Many important factors have to be 

 considered when selecting the individual objectives. Their 

 capabilities can only be determined with certainty by a very 

 careful examination of test objects, and by actual use in 

 photomicrography ; but objectives are now so carefully 

 made and tested that the claims of the maker as to their 

 working properties will usually be found to be well 

 sustained, and the probabilities are that the differences 

 between similar objectives of different makers of repute, 

 or of the same maker, can only be detected and their 



