LIGHT FILTERS AND SCREENS 79 



a set of two or tlirec noutrnl scrooiis to regulate the intcnisity 

 of the light. 



Red-stained objects usually photograph well with yellow- 

 green screens. High-power photographs of chromosomes 

 stained with iron-acetocarmine can be taken with Wratten 

 screen No. 56, using isochromatic plates. 



Practical Points 



1. Use fairly good plate glass for color filters, such as Wratten 

 C glass, when a corrected condenser is employed. 



2. Yellow-green light-filters may, with advantage, be used 

 continuously on the microscope. 



3. A special light bluish glass (Corning), or gelatin film 

 (Wratten), cutting out much of the red and yellow when used 

 with tungsten light, is excellent when it is required to view objects 

 in their natural colors (artificial daylight). 



4. Other color screens, not yellow-green, are mainly useful for 

 the photography of colored objects with apochromatic objectives; 

 though blue-green (or blue) screens allow of temporary vision, 

 or of photography, at increased aperture. 



5. Keeping the gelatin color filters in darkness, when not in 

 use, prolongs their usefulness. 



6. The plane surfaces of the filters should be kept fairly clean. 



7. Some means of altering the intensity of the light is essen- 

 tial for the attainment of maximum aperture. This can be done 

 with a series of yellow-green or neutral screens. 



8. The use of yellow-green screens is the cheapest way of 

 improving the definition and resolution of the microscope, 

 especially the achromatic microscope. 



