76 THE USE OF THE MICROSCOPE 



the eyes.) If kept in a box when not in use, these filters 

 last, in the writer's experience, for years without fading. 



Other Color Filters. — Other Wratten screens (62) of 

 different colors, not yellow-green, are of use chiefly for 

 short observations and for photographing colored objects 

 with apochromatic objectives. Minus red 4, however, is 

 preferred by Nelson for observation; and minus red 3, 

 No. 64, is sometimes used by the writer with good effect, 

 especially on the monocular. These are both blue-green. 

 Number 64 plus No. 56 gives a clear green. 



A shghtly bluish screen (Wratten No. 78), or a special 

 bluish glass (Corning Glass Company), gives the effects of 

 daylight with tungsten light (artificial daylight), and is 

 useful on colored objects not stained. Personally, the 

 writer prefers the yellow-green light on objects stained 

 with carmine, hsematoxylin, brazilin, or gentian violet. 

 Those who use white light for all work lose the special 

 advantages of yellow-green light. However, if the eyes 

 get tired of green, a change to artificial daylight is good. 



Advantages of Yellow-green Light. — The advantages 

 of constantly using yellow-green color filters are the 

 following : 



1. To rest the eyes. These filters (and sometimes a 

 blue-green one) have been used continuously by the writer 

 for this purpose, on the binocular microscope, several 

 hours a day, for over four years, with satisfactory results. 



2. To improve the corrections of the objective and con- 

 denser. It is well known that the spherical aberrations of 

 the achromatic and fluorite objectives are corrected for the 

 yellow to yellow-green. Hence the use of a yellow-green 

 screen improves the spherical corrections by cutting out the 

 badly corrected colors. Thus these screens strikingly 

 improve the performance of the high-power achromatic 

 and fluorite objectives. The yellow-green screens also 

 improve the color corrections of the achromatic objectives 

 by confining them to the middle of the spectrum. Even 

 the apochromatic objectives are somewhat improved by 

 the use of such yellow-green screens as Nos. 66 and 56. 



