368 OSCAR W. RICHARDS 



The microscope requires no special skill for its use. An auxiliary 

 telescope is used in place of the eyepiece while viewing the diffraction 

 plate in the objective for the purpose of centering the microscope con- 

 denser and the annular diaphragm. On replacing the eyepiece the in- 

 strument is ready for use. A microscope illuminator with a focusing 

 lens system is desirable and for greatest contrast monochromatic 

 light is preferable. For many materials and living cells good bright 

 contrast is obtained with a 0.2A -f- 0.25X diffraction plate and dark 

 contrast with a 0.2A — 0.25X plate. Increased contrast can be ob- 

 tained with the 0.14A + 0.25X and 0.14A - 0.25X plates and greater 

 contrast obtained with lower transmission plates of 0.07A ± 0.25X, 

 especially with very small specimens. The IB — 0.25X diffraction 

 plate is useful with supravitally stained preparations, Feulgen stains, 

 and with colored or absorbing material of low contrast. Sudan- 

 black-stained material shows with the greater dark contrast 2.5B — 

 0.25X plate, as do plant chromosomes and yeast cells. For special 

 applications plates of other than a quarter wavelength retardation 

 give better contrast {57). Some specimens show better with bright 

 or dark contrast, some equally well with either and some show detail 

 with one not seen with the other. Likewise some observers may pre- 

 fer to use bright or dark contrast. Dark contrast more nearly re- 

 sembles stained material and is helpful for measuring. Bright con- 

 trast seems better for counting. Bright contrast will reveal the pres- 

 ence of smaller objects when the intensity of the light is adequate and 

 is useful for stereophotomicrographs UOb). Color contrast may be 

 added to phase contrast (Fig. 9). Some experience with the equip- 

 ment is necessary for its efficient use and a proper understanding of the 

 image. 



Specimens with small optical path differences are better than those 

 with large optical path differences for phase microscopy. In fact 

 it is often possible to improve the contrast by varying the optical 

 path with media of slightly different refractive indices. Specimens 

 with large path differences or with good color contrast are better 

 examined w^th other microscopic methods. The phase microscope 

 has made possible the direct examination of both transparent and 

 low absorption contrast specimens that hitherto could be examined 

 only by staining and other indirect methods. Even though it has 

 been used only for a short period of time, advances are being reported 

 in biology, medicine, and industry (38-40,67,58,60,68). 



