152 INSTRUMENTATION 



relatively high reflectivity is also applied over the upper face along 

 MGN. Frangon used a film of titanium dioxide formed by means of 

 an evaporation process. The diffraction plate or its equivalent is placed 

 on the highly reflecting film and is centered on the optical axis of the 

 condenser-hemisphere-microscope system. 



The conjugate area of the diffraction plate may have the shape of 

 either a disk or an annulus. The diffraction plate may be constructed 

 by any of the known procedures, such as evaporating a suitable com- 

 bination of metallic and dielectric films, cementing films of plastic or 

 gelatin at the interface MGN, or using polarizing or birefringent materials 

 or both as in systems of variable phase microscopy. In Fig. III. 12 

 the conjugate area is designated by the letters HI. After the dif- 

 fraction plate has been added, the two parts of the hemisphere are re- 

 combined. The hemispherical surface, or whatever portion of a 

 hemispherical surface is required, is silvered except over the small 

 circular aperture CD centered on the optical axis of the system. The 

 specimen is adjacent to the plane AOF and is centered at 0. In order 

 to permit the use of a standard microscope preparation with a cover 

 glass 0.17 mm thick, the thickness of the lower part of the glass hemi- 

 sphere is reduced by 0.17 mm. For greater ease in moving the specimen 

 slide into the correct position and to prevent scratching, it is preferable 

 further to reduce the thickness of the lower part of the hemisphere by 

 0.1 or 0.2 mm and to use a film of oil between the slide and the hemi- 

 sphere, regardless of the numerical aperture of the microscope objective. 

 The cover glass and the oil film then complete the hemisphere. 



In Fig. III. 12 a ray of light emerging from the opening in the con- 

 denser diaphragm is incident on the specimen at the point 0, the center 

 of curvature of the hemisphere. Part of the light is deviated by the 

 object specimen. The broken lines represent rays associated with the 

 light deviated by the specimen, and the solid line within the hemisphere 

 designates the ray associated with that part of the light which is not 

 deviated by the specimen. Consider the deviated ray OP. This ray 

 is partially reflected and partially transmitted (ideally, half the light 

 energy is transmitted) at the dielectric film applied over the face at 

 MGN. The reflected part of the light does not enter the microscope 

 objective. The transmitted portion incident at P is reflected by the 

 silvered surface back on itself to the interface MGN, and partial re- 

 flection again occurs at the highly reflecting dielectric film. The ray 

 arising from this last reflection passes through the opening designated 

 by CD. In the same way, the other rays which represent the deviated 

 light waves originating at emerge through the aperture CD and inter- 

 sect at a point 0'. Similarly, a part of the undeviated ray which passes 



