LUni'r FII/I'KRS AM) SCREENS 77 



(Of course none of the color screens mentioned above approx- 

 imates to monochromatism; which is to be had, witli 

 maximum brightness, from the mercury lamp.) The 

 aspheric condenser is so much improved by Wratten deep 

 yellow-green screens, that it nearly equals the best achro- 

 matic condensers. 



3. To give greater contrast ; especially with objects stained 

 with carmine, hsematoxylin, or brazilin; and some other 

 red and blue or brown stains. The Wratten book on 

 photography with the microscope (61) explains how to 

 get the maximum contrast in photography with the 

 Wratten filters of various colors. There seems, however, 

 little need of using filters of many colors; since it would 

 seem more practical to stain the objects especially for 

 photography, and to choose for this the best colors, which, 

 with suitable yellow-green screens, come out black in 

 prints from isochromatic plates. 



4. To regulate the intensity of the light (which, of course, 

 cannot be done with the iris diaphragm of the condenser 

 without altering the aperture at the same time) a series 

 of yellow-green screens suffices, the screens being used 

 alone or in combination. They have the advantage over 

 neutral screens in that they cut out the optically detri- 

 mental rays of light, whereas the neutral screens cut out 

 the good with the bad rays. 



5. By cutting out the red and part of the yellow, as well 

 as the blue and violet, the visual aperture is probably 

 increased ; for the blue and violet do not seem so important 

 visually. 



6. To improve the acuity of vision by eliminating the 

 chromatic errors of the eye itself (Hartridge). 



Thus the use of yellow-green light filters is the least 

 expensive way of improving the microscope. If the eyes 

 should in time get tired of yellow-green, a temporary shift 

 to blue-green or to artificial daylight is the remedy. The 

 Wratten screens should be kept in a box when not actually 

 in use; for continuous exposure to strong light of course 

 slowly fades them. The Chance and the Jena glass works, 



