CHAPTER IX 



COLOUR-SCREENS AND PHOTOGRAPHIC PLATES- 

 EXPOSURE 



Colour-screens. In photographing microscopic objects it will 

 almost invariably happen that a colour-screen is necessary. It 

 is either useful when using achromatic objectives to eliminate 

 that portion of the spectrum for which the objectives are not 

 corrected, and so to obtain a photograph which does not suffer 

 owing to the want of colour- correct ion of the objective, or it 

 may be necessary to increase or reduce colour-contrast in the 

 object itself. It will very soon be found that although an 

 object may have sufficient contrast visually, it by no means 

 follows that in photographing this object the same contrast 

 can be obtained on the photographic plate. 



The elimination of the harmful portion of the spectrum, so 

 far as the objective is concerned, is a comparatively simple 

 matter, and can usually be secured by using a yellow screen 

 which cuts out the blue and violet portions of the spectrum. 

 The part so cut out is, of course, the photographically active 

 portion ; but, as in conjunction with such a screen a yellow 

 sensitive plate would be used, this does not result in such great 

 lengthening of the exposure as would otherwise be the case. 

 To increase the colour-contrast of a preparation is not such a 

 simple matter, and the only effective way is to adapt the screen 

 to the particular dye, or combination of dyes, with which the 

 preparation is stained. Colour-screens may now be purchased 

 with almost any absorption spectrum, cutting out, therefore, 

 almost any part of the spectrum which it is wished to elimi- 

 nate. To purchase such a set of screens is a somewhat 

 costly matter, and many workers may hardly feel justified in 

 incurring such an expense. Fortunately they may be made up 



205 



