104 INSTRUMENTATION 



corporated in the proper area of the diffraction plate in order to obtain a 

 favorable amplitude transmission ratio between the conjugate and 

 complementary areas. Decreasing the width of the conjugate annulus 

 decreases the amount of absorption ordinarily required in the conjugate 

 area. 



The modification described by Grigg (1950) will also serve as a method 

 for obtaining reversal color contrast. In this scheme, the opaque area 

 of the standard condenser diaphragm for phase microscopy is replaced 

 by a color filter, and a filter of contrasting color is mounted over the 

 normally transmitting area of the diaphragm. This bi-colored dia- 

 phragm is used together with the standard, neutral diffraction plate. 

 Suppose that the area of the phase condenser diaphragm which normally 

 transmits all the incident illumination and which is imaged on the 

 conjugate area of the diffraction plate is now made to transmit light of 

 color A. Suppose also that the normally opatiue region of the phase 

 condenser diaphragm is replaced by a filter which transmits light of 

 color B. This area of color B is imaged on the complementary area of 

 the diffraction plate. Assume that the diffraction plate is a general- 

 purpose dark-contrast plate. Then, if a particle having an optical 

 path exceeding that of the surround is moved into the field of the 

 microscope, the diffraction plate again functions as a dark-contrast plate 

 for the light of color A. Since for the light of color B the conjugate and 

 complementary areas of the diffraction plate have in effect been inter- 

 changed, a bright-contrast image of the particle is formed with the light 

 of color B. The image of color B is superimposed upon the black image, 

 so that the particle appears color B. The halo appears color A, and the 

 general surround appears some color intermediate between A and B. 

 Similar reasoning will show that, if the particle has an optical path 

 smaller than that of the surround, the color of the image of the particle 

 is A, the color of the halo is B, and the color of the image of the surround 

 is intermediate between A and B. Too much chromatic overlapping 

 of the substage condenser filters should be avoided. With this method 

 and the standard diffraction plate, the intensity of the light admitted 

 by the normally opaque area of the substage condenser diaphragm must 

 be less than that admitted by the normally transmitting area. Polar- 

 oids mounted over the proper regions allow control of this intensity. 



2.5. Nomenclature for dilTraction plates 



As the variety of available diffraction plates increases, a generally 

 accepted nomenclature for designating the diffraction plate with which a 

 particular observation is made will become increasingly important. The 

 essential characteristics of a diffraction plate are fi^ = T, the ratio of 

 the energy transmission of the conjugate area to the energy transmission 



