350 OSCAR W. RICHARDS 



(c/. Sect. Al) at two wavelengths or colors and spherical aberration at 

 one wavelength. They are satisfactory for ordinary microscopy. 

 The further correction of aberrations in the objective requires the use 

 of fluorite as well as glass. Apochromatic objectives are corrected 

 chromatically for three colors and spherically for .two colors. The 

 fluorite, or semiapochromatic, objectives are of intermediate correc- 

 tion. Apochromatic objectives give better images and are desirable 

 for the examination, and especially for the photomicrography, of 

 colored specimens. Apochromatic objectives require achromatic- 

 aplanatic condensers and compensating oculars for best image forma- 

 tion. 



Limiting the light with a suitable filter to the yellow-green will 

 give a better image with achromatic objectives (when this color 

 limitation is not objectionable), because that is the region for which 

 the objectives are corrected and also the region of greatest sensitivity 

 of the eye. 



The contrast in the image of colored specimens may be increased 

 or decreased by varying the color of the light used to illuminate the 

 microscope with filters. Specimens may be stained with several dif- 

 ferent colors for greater differentiation of structural detail. The 

 methods are well known and described in reference books (£,4,^). 

 Increased color contrast may also be obtained by optical staining and 

 with phase microscopy (Sect. E3, Sect. I). Color is also visible 

 when the preparation is observed with the darkfield microscope (Sect. 

 El) when contrast of the color against a dark background is desirable. 



Color filters are available in glass and gelatin to meet most needs 

 {61,62). Tungsten light contains a greater proportion of the longer 

 wavelengths than sunlight and may be changed to daylight quality by 

 filtering it through a blue glass. The nature and thickness of the 

 filter must be balanced to the color temperature of the lamp and the 

 lamp maintained at the design voltage. For microscopy, light corre- 

 sponding to the temperature range 4500 to 5000° K. gives good color 

 rendition. For natural color photography, color-compensating filters 

 are usually necessary and the color should be made correct at the 

 plane of the film or plate (7) . 



The disadvantages of staining specimens for observation arise 

 from the time and materials required and the fact that most living 

 organisms are injured or killed, since very few stains are nontoxic 

 enough to be used on living material. Some specimens cannot be 

 colored with solutions of dyes while others require mordanting or 



