INTEKIERENCE >IICKOSCOPY 



9. Dysox, J., Proc. Roy. Soc. (London), A216, 

 493-501 (1952). 



10. Krvg, W. and Lau, E., Ann. Phys., 6 (8), 



329 (1951). 



11. Zehender, E., see Kohaut, A., Werkst. v. 



Betr., 86 (12), 725-732 (1953). 



12. Dyson, J., Engineering, 179, 274-276 (1955). 



13. Dyson, J., Phijsica, 24, 532-537 (1958). 



14. Davies, H. G. and Wilkins, M. H. F., Nature, 



169, 541 (1952). 



15. Barer, R., Nature, 169, 366 (1952). 



16. Davies, H. G., and Deeley, E. M., Exp. 



Cell Res., 11, 169 (1956). 



17. Mitchison, J. M., Passano, L. M., and Smith, 



F. H., Quart. J. Micr. Sci., 97, 287 (1956). 



18. Hale, A. J., "The Interference Microscope in 



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J. Dyson 



PLASTICS. See GENERAL MICROSCOPY, p. 390. 



PULP AND PAPER. See GENERAL MICROS- 

 COPY, p. 394. 



THEORY AND TECHNIQUES 



Although interference of Hght waves has 

 long been used for high precision measure- 

 ments in physics and technology, only 

 recently has it been applied to any large de- 

 gree in microscopy. The invention and wide- 

 spread success of the phase microscope em- 

 phasized the point that much could be gained 

 by application of the principles of physical 

 optics. One result has been the development 

 of a number of interference microscopes 

 which allow three dimensional study of ob- 

 jects, both qualitatively and quantitatively. 



Opaque objects can be examined by inter- 

 ference microscopes employing reflected 

 light. Transparent objects, normally in- 

 visible in an ordinary microscope, can be 

 studied by transmitted light. As in the phase 

 microscope, this permits living material to 

 be examined without staining or other special 

 preparation. 



Both the phase and interference micro- 

 scopes function by causing light waves to 

 interfere. In each the waves are first split 

 apart so that they follow different physical 

 paths; they are then treated differently; and 

 finally they are brought together to inter- 

 fere. With the phase system irregularities in 

 the object itself cause some of the light to 

 deviate from its original direction. These de- 

 viated rays are then treated differently by 

 the phase plate than are the undeviated 

 rays. Thus the resulting image is character- 

 istic of irregularities and discontinuities in 

 the specimen. 



In the interference microscope the light 

 ray separation is accomplished by means of 

 a beam-splitter. The object modifies one 

 beam with respect to the other by retarding 

 or advancing it. The image which results 

 when the beams are recombined is therefore 

 characteristic of the light-retarding proper- 

 ties of the object. Aii area of uniform optical 

 path appears with a uniform brightness, or 

 if white light is used the color is constant. 



The really distinctive feature of the inter- 

 ference microscope is that it allows measure- 

 ments of the optical path to be made. From 

 these measurements information on the 

 thickness of objects, the index of refraction 

 of solids and liquids, the concentration of 

 protein solutions, and the mass of cells can 

 be deduced. 



Interference of Light 



The constructive and destructive inter- 

 ference of waves, illustrated in Fig. 1, 

 follows certain fundamental laws. First of 

 all, the two beams must be coherent, which 

 in practice means that they must have come 

 from the same source. Second, they must 

 have the same wavelength. Third, if the 

 beams are plane polarized, they will not 

 interfere when the planes of polarization are 

 mutually perpendicular, but only when they 

 are brought to the same plane. 



The concept of a wavefront is very useful. 

 For the common plane wave, the wavefront 



420 



