INTEHFE HENCE MICKOSCOPY 



tical path than the surround will appear 

 bright. If the analyzer is turned until it is 

 perpendicular to the plane of polarization in 

 the image of the specimen, the specimen 

 will have a minimum of luminance. The ori- 

 entation of the analyzer is then read from a 

 scale to determine the optical path in the 

 specimen. 



As in the other two-beam systems, if 

 white light is used the specimen appears in 

 color contrast. The colors change as the 

 analyzer is turned. This is often useful in 

 studying and photographing complex speci- 

 mens. 



For measurement of optical path, mono- 

 chromatic light is necessary. The half- and 

 quarter-wave plates are made for the green 

 radiation emitted by a mercury arc. This 

 wavelength, 546 m/x, was chosen partly be- 

 cause it is very near the peak of spectral 

 sensitivity of the eye and partly because 

 convenient, very bright mercury arcs are 

 available and the green radiation can be iso- 

 lated easily by means of gelatine, glass, or 

 interference filters. 



The human eye has great sensitivity for 

 comparing the luminance of adjacent por- 

 tions of a field of view. In order to take ad- 

 vantage of this sensitivity half-shade eye- 

 pieces have been designed for use with this 

 polarizing type of interference microscope 

 (21, 22). For half -shade eyepieces employ- 

 ing the optimum half-shade angle (23) and 

 used with the 100 X shearing system, ana- 

 lyzer settings reproducible to 0.5° standard 

 deviation can be obtained on specimens with 

 areas of uniform path difference. 



The object is set so that its image strad- 

 dles the dividing line. As the analyzer is 

 turned the two halves of the specimen image 

 change in luminance relative to each other. 

 At the match setting, the analyzer orienta- 

 tion, ^1 , is noted. Then the analyzer is 

 turned until a match is obtained in the back- 

 ground adjacent to the specimen, and the 

 reading, 02 , is noted. 



The optical path difference between the 



specimen and surround is computed accord- 

 ing to the eciuation 



9i — Oo 



<j, s OPD = X 



^ 180 



where X is the wavelength of the light used. 

 6 1 and d-i are in degrees, is in units of length 

 e.g., millimicrons. The same ec^uation ap- 

 plies for the analyzer orientations di and 6^ 

 determined by the half -shade method or by 

 the extinction method described earlier. 



If the object has a path difference greater 

 than one wavelength, the above procedure 

 will yield only the fraction by which (j) is 

 different from one, two, etc., full wave- 

 lengths. To determine the correct order 

 number a convenient method is to use a 

 quartz wedge eyepiece and white light illumi- 

 nation. Colored fringes cross the field, one of 

 which appears blackest. The wedge is moved 

 across the field and the black fringe is seen 

 first in the specimen and then in the adja- 

 cent surround. The number of full wave- 

 lengths of path difference in the specimen is 

 equal to the number of dark fringes which 

 pass the specimen in this operation. For spec- 

 imens with considerably different disper- 

 sion than the surround, the precautions men- 

 tioned by Faust and Marrinan (24) should 

 be observed. 



Franqon Interferential Eyepiece (25). This 

 system, like the multiple-beam method, per- 

 mits interference contrast to be obtained 

 with an ordinary microscope. An illuminated 

 slit below the condenser is the effective 

 source. 



In the eyepiece, light of one polarization 

 is sheared with respect to light of the oppo- 

 site polarization. This could be accomplished 

 by a single plate of birefringent material, 

 as shown in Fig. 2c, except that there is 

 then a large path difference between the 

 beams and they cannot interfere, at least 

 with the light sources normally used in mi- 

 croscopy. Equalization of path differences is 

 accomplished by using two plates of equal 

 thickness as shown in Fig. 9a. The combi- 



428 



