THi:OKY AND TECHNIQUES 



mitted and partially reflected at each metal- 

 lized surface, as shown in Fig. 4a. 



In areas where the optical path difference 

 between successive transmitted rays is an 

 integral number of wavelengths, construc- 

 tive interference occurs, and a bright fringe 

 is seen in the field. A single fringe therefore 

 traces out a contour line of equal optical 

 path, and the deviation of a fringe as it 

 passes through a specimen can be used to 

 measure the optical path of the specimen. 



The higher the reflectance of the metal- 

 lized surfaces, the sharper is each fringe (6). 

 For biological work with living specimens, an 

 important requirement is that non-poison- 

 ous metals such as mconel or titanium be 

 used. If the index difference between speci- 

 men and mounting medium is considerable, 

 additional multiple reflections may occur 



which add complexity to the fringe pattern 

 (7, 8). 



For the examination of surfaces and steps 

 on flat surfaces the system can be used in 

 reflection, through the use of a vertical illu- 

 minator. If the surface to be tested does not 

 have sufficient reflectance it can be over- 

 coated with silver, which contours the sur- 

 face (9). For highest precision, such as in 

 measuring glass or metal polishing defects 

 or the thickness of evaporated films, fringes 

 of equal chromatic order are often u.sed (10, 

 11). As shown in Fig. 4b, a white light 

 source is used and the image of the specimen 

 is projected onto the slit of a spectrograph. 

 The continuous spectrum is crossed by dark 

 fringes, and the displacement of a particular 

 fringe as it crosses the specimen is a direct 

 measure of its physical height. Precisions of 



c 



D 



a. 



H 



Slow 



Fig. 3. Effects of wave-plates on plane-polarized light, (a) Half-wave plate, C, rotates the plane of 

 polarization from azimuth - a (at A) to azimuth + a (at E). At B are shown the components of the 

 incident light which are parallel (solid wave) and perpendicular (dashed wave) to the slow axis of the 

 crystal. At D the parallel component has been retarded one half wavelength relative to the perpendicu- 

 lar component. The sum of these two vibrations gives a vibration which lies along the line shown at E. 

 (b) A quarter-wave plate, H, converts linearly polarized light at F to circularly polarized light, shown 

 at J. The two components of incident light, shown at G, are equal and in phase. After passing through 

 the waveplate the components are out of phase by one quarter wavelength (I). As these two waves 

 cross a reference plane, their instantaneous sum can be found by vector addition as shown by the arrows 

 at I. As the wave progresses across the reference plane the tip of the vector traces out a circle, as shown 

 at J. 



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