150 INSTRUMENTATION 



projector system is designed to minimize spherical and chromatic aber- 

 ration in the plane of the exit pupil in which the diffraction plate is 

 located. An ordinary ocular is not sufficiently well corrected to sub- 

 stitute for the projector system. A beam-splitting cube follows the 

 projector system and diffraction plate. This cube allows a phase con- 

 trast image to be transmitted to the camera and also provides a re- 

 flected image for visual observation with an ordinary microscope eye- 

 piece. 



7. PHASE MICROSCOPY WITH ULTK AVIOLET AND INFRARED IL- 

 LUMINATION 



In recent years there has been much interest in designing microscope 

 and condenser systems for use with ultraviolet and infrared illumination. 

 Such optical systems may include either refracting or reflecting com- 

 ponents or both. The resolution of a given objective increases as the 

 wavelength of illumination decreases. In addition to higher resolution, 

 a further advantage may be gained with ultraviolet microscopy because 

 many substances that are transparent to visible light absorb ultraviolet 

 wavelengths selectively. The fact that many substances absorb in- 

 frared light selectively also makes infrared microscopy valuable. When 

 small optical path differences and small amounts of absorption are 

 present in the object specimen, a phase objective can increase the con- 

 trast in the image. However, determination of the absorption ratio 

 between two parts of an object specimen, which in the general case 

 contains both absorption and optical path differences, is a compli- 

 cated procedure that requires a system of variable phase microscopy 

 (Osterberg and Pride, 1950). 



Bennett, Woernley, and Kavanagh (1948) have reported some initial 

 work with an ultraviolet phase microscope. The light forming the 

 image of the specimen passes through the diffraction plate. If the dif- 

 fraction plate is of the transmission type, the same general design con- 

 siderations hold as for visible illumination except that the high absorp- 

 tion of ultraviolet radiation by many materials may impose an additional 

 limitation. If reflecting optics are contained in the objective, one of 

 the reflecting surfaces can be altered to form a reflection type diffraction 

 coating. If a coating is applied over part of the reflecting surface to 

 form an abrupt step, then light reflected from the step and that re- 

 flected from the adjacent area travel through different lengths of path 

 and a phase difference is introduced between such rays of light. The 

 reflectivity of the step can be made different from that of the adjacent 

 area, for example, by depositing coatings of different materials on the 

 two regions, so that any ratio of the light energy reflected from the step 

 to that reflected by the adjacent area can be obtained. 



