98 INSTRUMENTATION 



color diffraction plates to be discussed in this section will be called the 

 simple color phase or diffraction plate and the reversal color diffraction 

 plate. The simple color diffraction plate does not change the phase 

 difference between that part of the deviated and the undeviated light 

 which includes the wavelengths near one end of the visible spectrum, 

 but it does introduce a relative phase change of approximately 90° 

 between that part of the deviated and the undeviated light which in- 

 cludes the wavelengths at the other end of the visible spectrum. If a 

 reversal color diffraction plate is mounted in the objective, bright .con- 

 trast is obtained if the illumination incident on the specimen is limited 

 to wavelengths that lie near one end of the visible spectrum, dark con- 

 trast is observed if the wavelengths lie near the other end of the visible 

 range, and no phase contrast occurs if the light incident on the speci- 

 men consists only of a band of wavelengths at some intermediate region 

 of the visible spectrum. 



For example, consider a simple color diffraction plate designed to pro- 

 duce dark contrast if the object specimen has an optical path greater 

 than that of its surround and if it is illuminated with light containing the 

 yellow, green, and blue wavelengths. With the same object specimen 

 consider also a reversal color diffraction plate designed to produce bright 

 contrast if red illumination is used, dark contrast if blue or green light 

 only is incident on the specimen, and no phase contrast if yellow light 

 illuminates the specimen. References to the color of the image of the 

 specimen should be interpreted only as meaning that the color is pre- 

 dominantly or approximately that mentioned. If the source of light is 

 white and if the phase objective contains a simple color diffraction coat- 

 ing, the image of a particle with an optical path greater than that of the 

 surround is red. If a green filter is placed in front of the light source 

 the image of the specimen is dark or black, but if a red filter is placed 

 in front of the light source the image of, the particle is hardly distinguish- 

 able from that of the surround. If the diffraction plate or coating in the 

 objective is of the reversal color type, then the image of the same particle 

 is again red if a white light source is used. If a red filter is in the light 

 beam the image of the specimen appears brighter than the image of the 

 surround, but if a green filter is used the image of the specimen is dark 

 or black. 



If, in this example, a particle has an optical path less than that of the 

 surround, then all the color contrast phenomena are reversed and the 

 following effects are observed. The combination of the phase objective 

 with the simple color plate and a white light source produces a greenish 

 image of the particle. The addition of a green filter at the light source 

 causes the particle to appear brighter than the surround, but the particle 



