190 THE TECHNICS OF PHASE MICROSCOPY 



contrast 0.14A±0.25X and 0.07A±0.25X plates often require two to four 

 times the equivalent exposure required by brightfield. Other makes of 

 phase equipment with narrower conjugate areas rerjuire correspondingly 

 longer exposures. These figures are suggestive only and are given to 

 assist the microscopist in determining the proper exposure. More 

 exact exposures can be obtained with exposure meters appropriate for 

 photomicrography (Maurer, 1944). With colored light and color 

 filters the exposures are increased accordingly by the usual factors. 

 Even with the best eciuipment some experience is necessary, and trial- 

 and-error methods may be required for exposure determination. 



Stereophotomicrographs are helpful in showing the third dimension 

 of a specimen and may be made by several methods. With low powers 

 the specimen may be tilted about 7° and one picture made, then tilted 

 the same amount in the opposite direction and the second picture of the 

 stereopair photographed. With higher magnifications there is not 

 enough depth of field for this procedure, and the half-aperture method 

 is satisfactory provided that the loss of half of the resolution is of no 

 consequence. A piece of black Scotch tape is placed to cover one-half 

 of the annulus, and the first picture made. The annulus is rotated 

 180°, or the tape is removed and placed on the opposite half of the 

 annulus, and the second exposure is made. The simple condenser is 

 more convenient for this than is the turret condenser. The two ex- 

 posures of the pair should be processed together and mounted so that 

 corresponding points are about 2.5 inches apart and so that horizontal 

 lines in each are in line to avoid tilt. The mounted pair may be viewed 

 with a stereoscope for study of the third dimension (Fig. IV. 6) or, after 

 some practice, fused on direct observation. The picture should be held 

 so that by looking over it one may see some distant object. When the 

 eyes are quickly dropped to the pictures, the pictures will fuse into one, 

 with tridimensional depth. One should look through rather than at the 

 pictures so that the accommodation will be relaxed. 



The. important precaution in phase photomicrography is to make sure 

 by checking with the telescope that the image of the annular diaphragm 

 is concentric with and superimposed on the diffraction plate. Any 

 light leakage will markedly decrease the contrast in the photograph. 

 Light leakage around the conjugate area will spoil the definition of the 

 photomicrograph more than leakage around the outside of the objective, 

 although both should be avoided. Glare from light leakage is additive 

 in photography because the camera cannot focus and adapt like the eye. 

 Proper alignment should not be neglected for efficient visual microscopy. 

 For adequate photomicrography the condenser must be centerable and 

 the annuli for the various apertures also must have individual centering 



