132 INSTRUMENTATION 



to the eye. With dark-contrast diffraction plates which do not have 

 sufficient absorption in the conjugate area to cause reversal, detail K 

 appears darker than the edge of .4. Detail K also appears darker in the 

 non-phase photomicrographs, and a tiuestion arises concerning the 

 absorption and optical path of this detail as compared with the optical 

 properties of the main body of oil pool A. Similarly, the non-phase 

 photomicrographs and most of the phase photomicrographs show very 

 small areas of higher contrast in groups L, M, N, H, P, and R, and there 

 is the suggestion of some absorption in these areas. The difference in 

 optical path between a particle and its surround as well as the relative 

 transmission of the particle can be determined by means of a variable 

 phase microscope which allows continuous variation both of the differ- 

 ence in optical path and also of the transmission ratio between the 

 conjugate and complementary areas of the diffraction plate. The size 

 and shape of the particle will also affect the amount of contrast in the 

 image. 



In the discussion to follow, these small areas of darkest contrast that 

 are observed within the images of the oil pools will be referred to as 

 inclusions although the observations reported in this section offer no 

 conclusive evidence concerning whether the increase in contrast indi- 

 cated a change in optical path alone or whether some absorption was 

 also present within the so-called inclusions. It is possible that tiny 

 pieces of iron became trapped under the magnesium fluoride film so that 

 iron or iron oxide particles introduced some absorption. That the 

 inclusions constituted areas of greater optical path only, with no absorp- 

 tion, can be accounted for by assuming that during the evaporation the 

 iron filings adhered most closely to the slide over these areas and there- 

 fore shielded these regions more completely from the magnesium 

 fluoride. 



High-contrast diffraction plates were selected to observe changes in 

 the extent of the halo around each particle and in the extent of the dark- 

 ening in the image of a particle as the dimensions of the conjugate area 

 were changed. If an object specimen contains an inclusion which 

 appears darker than the remainder of a specimen when the diffraction 

 plate in the objective produces a high-contrast, dark image of this 

 specimen, it is sometimes possible to enhance the visibility of the 

 inclusion with respect to the whole object specimen by substituting 

 a diffraction plate which has a higher transmission in the conjugate area. 

 Therefore photomicrographs were taken also to show the change in the 

 visibility of the inclusions with respect to the edges of the oil pools as 

 the energy transmission only of the conjugate area of the diffraction 

 plate was changed. 



