170 THE TECHNICS OF PHASE MICROSCOPY 



holder. A lamp with a CC-13 coiled-coil tungsten filament will, when 

 properly focused, normally give greater and more uniform illumination 

 than other types of coiled filaments, and a 100-watt T-8 50-hour pro- 

 jection lamp provides brighter illumination than a 200-hour G-16.5 lamp 

 or a ribbon filament lamp. The uniformity of illumination of the rib- 

 bon filament offers slightly better definition although it is less bright. 



The lamp should be positioned in the illuminator by adjusting it, or 

 the reflector, until the reflected images of the coils are intermeshed with 

 the direct image to give as uniformly illuminated an area as possible. 

 This adjustment can be done by looking at the filaments directly after 

 removing the lens system, or by closing the lamp iris and focusing the 

 filaments on a nearby wall, or other convenient surface. (The mirror 

 is usually omitted from the illuminator with a ribbon filament lamp, 

 making this step unnecessary.) With both, it is important that the 

 filament be centered to the lens. 



Direct the light beam from the illuminator onto the center of the 

 microscope mirror. (This is readily accomplished if the mirror is 

 turned to reflect the beam on the lamp iris.) Turn the mirror to reflect 

 the light into the microscope, and focus the lamp so that its filament image 

 is in focus on either the annulus or the diaphragm of the microscope 

 condenser. Focus the microscope condenser so that the lamp iris is in 

 focus with the specimen, and open the lamp iris to illuminate the part 

 of the field to be studied. If the light is too bright for comfortable 

 vision, it may be reduced by placing neutral density filters between the 

 lamp and the microscope. Since most glass filters are not quite neutral, 

 a ground glass may be used in place of the neutral filter to reduce the 

 illumination, and this is permissible for non-critical work. For the 

 finest detail and best photomicrographs a ground glass should be 

 avoided. Raising the microscope condenser slightly will diffuse out the 

 pattern of the ground glass without unduly reducing the uniformity of 

 illumination. 



Illuminators consisting of a lighted surface with adequate intensity 

 for comfortable vision may be used, although as good detail and contrast 

 should not be expected as from the illuminator with focusable lens 

 system. Units with built-in illumination are adjusted according to in- 

 structions of the manufactvu-er. In all cases the field and aperture 

 should be lighted uniformly and the image of the condenser annulus 

 should match the corresponding part of the diffraction plate with no 

 overlapping or leakage of light. 



Strictly speaking, a diffraction plate has a given retardation only for 

 the wavelength of radiation for which it is designed. A quarter-wave 



