XI. WHEN TO USE SPECIAL MICROSCOPES 



367 



Light waves passing through an optically denser medium as at C in 

 Figure 10 are slowed with respect to those not passing through the denser me- 

 dium, but the difference does not affect the eye or photographic plate. An ab- 

 sorption difference such as E does not alter the phase relations but changes 

 the amplitude and is visible. An annular tliai)hragm, K, is placed in the 

 condenser of the phase microscope and an image of its illuminated opening is 

 formed at the back focal plane, J', of the objective. At this plane is placed 

 a diffraction plate which may consist of either {1) a thin layer of metal 

 and/or dielectric, as at F, of proper size to cover this image of K (conjugate 

 area), or {2) a complementary area of similar composition as depicted at /, 



Annular diaphragm Dielectric 



€1^ Diffraction plate / Light waves 



k i I HG F / A 



!/l\/!\/V7VA /?!! ' 



kMetal ^/WW^' 





P CD<< 





o' 



N 



P 



Fig. 10. Diagram of the phase microscope {57). 



or (S) some desired combinations of the / and F type distributions as shown 

 at G and H. When a specimen is placed on the phase microscope it diffracts 

 light; this deviated light passes through the entire objective, while the un- 

 deviated light must pass only through the conjugate area. The diffraction 

 plate thus has a differential effect on the light. When the image is formed by 

 the combination of all the separate rays from the specimen and its back- 

 ground, light vibrations of the same phase and amplitude, R, add to give in- 

 creased brightness; likewise those of opposite phase and amplitude, S, pro- 

 duce darkness. Intermediate phases and amplitudes combine to give grays. 

 Diffraction plates of the A+ and B+ types give bright contrast, as regions 

 of greater optical path appear brighter than those of lesser path and con- 

 versely, the A— and B— plates show the regions of greater path as darker. 

 By changing plates having different thicknesses of absorbing metal and/or 

 retarding dielectric layers, the image contrast may be changed to optimum 

 for any given specimen. Further explanation of the theory requires mathe- 

 matical analysis, which is available elsewhere together with a history of the 

 method {39). 



