ILLUMINANTS 171 



plate for the 546-mju mercury green line, for example, will be about 0.3 ju 

 at 400 niju and 0.2 ^t at 700 m^, and for greatest contrast monochro- 

 matic light should be used. When color is not objectionable, the 54G-mM 

 line may be isolated with a Wratten No. 62, or equivalent, filter from a 

 mercury arc; or the sodiiun arc may provide monochromatic sources. 

 For visual observation, nearly as satisfactory results may be obtained 

 with an Interference Filter with a tungsten lamp (preferably one at 

 555 m/x, the peak sensitivity of the eye). 



In actual experience the contrast is related to the absorption, the 

 retardation, and the width of the annulus of the diffraction plate (cf. 

 Sections 2-5 of Chapter III). With an annulus of sufficient width to 

 give adequate halftone detail in a properly adjusted objective the gain 

 from restricted wavelength is not so great as might be expected, and the 

 contrast and detail in the image do not vary a great deal with the use of 

 the usual medium broad-band photomicrographic filters such as com- 

 binations of the Wratten M filters, or their eciuivalents of other manu- 

 facture. Hence this is not a promising method for obtaining variable 

 phase. The result is partly explained by the decreased sensitivity of 

 the eye to wavelengths on either side of the yellow-green. With nar- 

 rower annular widths, less complete correction of the objective, or both, 

 restricting the light to a limited wavelength region (such as green) may 

 be desirable when such color limitation does not interfere with successful 

 observation. A more efficient filter with narrower transmission than 

 that of the Wratten B type would be appropriate. Phase microscopy 

 with slightly colored specimens or when color contrast is added to den- 

 siphase contrast precludes the use of color filters. No filter is required 

 by some makes of phase eciuipment. There is no objection to any 

 colored light that is pleasing to the microscopist and that does not de- 

 limit observation. Although maximum contrast can be obtained with 

 suitable monochromatic light, most visual phase microscopy can be done 

 with a standard microscope lamp and a daylight type of filter correct- 

 ing the light to a color temperature of about 4800° K. 



For some applications very intense sources may be reciuired. For 

 example, with bright contrast, the particle may be too small for the 

 resolving power of the instrument, and its discovery will depend on 

 having an illuminant bright enough to give a diffraction pattern illumina- 

 tion great enough to stimulate the eye or other recording medium. 

 Photomicrographic illumination will l)e discussed later in this chapter. 



The increased detail and contrast of the phase microscope require 

 magnifications of about 1000 times the numerical aperture of the ob- 

 jective for full visibility with the average eye, and greater magnification 



