VARYING THE CONJUGATE AREA 



131 



uniformly dark depends on the numerical aperture included in ^he 

 conjugate area of the diffraction plate as well as on the transmission of 

 the conjugate area. Particles of the size and shape of those included 

 in groups H and P appear uniformly dark with all diffraction plates, 

 regardless of the size of the conjugate area. 



R 



■o 



A^ .. 



^ 



Fig. III. 6. Drawing of the object specimen consisting of oil pools surrounded by a 



film of magnesium fluoride. The blackened areas indicate those regions of greatest 



contrast which have been referrred to as inclusions. 



In addition to these larger oil pools there is smaller detail such as 

 groups K (close to the upper right-hand edge of .4), L (just to the left 

 of the upper edge of .4), R, M, and N. These groups contain particles 

 which are so small as to be sensitive to loss of resolution and which show 

 reversal of contrast when the absorption in the conjugate area is high. 

 The four particles K may not be visible in the reproduction of the phase 

 photomicrographs although they were in every case readily discernible 



