CHAPTER IX 



COLOUR SCREENS AND COLOUR SENSITIVE PLATES 



Light Filters or Colour Screens. It has been already 

 pointed out that the complexity of white light and the 

 different refrangibilities of its component rays lead to 

 defects in the image formed by achromatic objectives, 

 and attempts to take photomicrographs with ordinary 

 white light will soon convince the worker that, in many 

 cases, other defects occur which cannot be referred to 

 these causes. Besides a possible lack of sharpness, due 

 to the imperfect corrections of the lenses, there may be a 

 lack of contrast or of differentiation in the parts of a 

 preparation, or of an object from the field in which it 

 lies, and no care in the adjustment of the illuminating 

 system or in the use of the most perfectly corrected 

 lenses will suffice to give a perfect photomicrograph. 

 The insertion of a piece of coloured glass in the system 

 between the radiant and condenser very frequently pro- 

 duces a vast difference in the resulting negative. The 

 IBS acts as a " Light filter " or " Colour screen," and 



)nly allows a certain portion of the rays emitted by the 

 radiant to pass through it. The photomicrograph is 

 therefore taken with light of a selected colour only 

 ?en, for example and becomes sharp, instead of slightly 



>ut of focus in appearance, on account of the improved 

 performance of the objective under these conditions. 

 Better contrast and increased resolution may also be 



tpparent in the negative, and in fact properly selected 



jolour screens are practically indispensable in photo- 



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