COLOUR SCREENS 161 



placed between the two glasses. Fig. 49 illustrates two 

 of these, and another in which the glasses are bound 

 together in a zinc frame with aquarium cement. 



The Use of Colour Screens. The main objects for which 

 it is necessary to modify the colour of the light by light 

 filters are to ensure : 



1. Good definition and resolution. 



2. Adjustment of contrast in the photomicrograph. 



3. Correct colour rendering. 



Good definition and Resolution. The connection be- 

 tween resolution and wave length was mentioned on 

 page 61. and the improvement produced in a photomicro- 

 graph by using blue or green light was noted. Green 

 screens are usually the best for this purpose as the 

 spherical corrections of the objective are most accurate 

 for green light, and also because the light they transmit 

 is more luminous than that passed through a blue screen, 

 and enables an accurate focus to be obtained more 

 readily. 



Adjustment of Contrast. This is often the most im- 

 portant use to which colour screens are put. However 

 accurately an exposure may have been gauged, and in 

 spite of the most careful use of the best optical equip- 

 ment, there is often a lack of contrast in the negative 

 between the object and background unless a suitable 

 colour screen is used with a suitable plate. This is 

 generally the case if the object possesses only a faint 

 colour, or is faintly stained, particularly if the stain is 

 blue. 



On the other hand, the contrast between the back- 

 ground and the object may be excessive, but detail in 

 the object lacking. This is most marked in deep yellow 

 preparations or in those objects which have been stained 

 a deep red ; and occasionally the contrast between 

 different parts of an object requires reduction. 



It is frequently desirable to emphasise certain parts 



