6 



INTRODUCTION TO PHASE MICROSCOPY 



for those who wish, a readily understandable explanation ^vill be given 

 here. 



Phase microscopy is based on the effect of the combining of light waves. 

 The properties of these waves on which phase microscopy depends are 

 amplitude and phase. Let us consider a small, transparent specimen 

 located on the optical axis of the microscope optical system. We will 

 assume that this specimen is an ideal one for phase microscopy. Hence, 

 we will embed this specimen in a medium having almost the same index 

 of refraction as the specimen. As the specimen is transparent it will 



P = D + S 



Fig. I.l. Retardation of a light wave by a particle (P) of greater index of refraction 



than its surround (S). 



differ from the surrounding medium in only one particular, i.e., in re- 

 fractive index, and this difference gives rise to a slight difference in 

 optical path between the specimen and its surroundings. Let us think 

 of a single light wave incident on the specimen and its surrounding 

 medium. In Fig. LI the sine curve, S, represents the light transmitted 

 by the surround; P is the wave transmitted by the particle. If the 

 particle has a slightly higher index of refraction than that of the sur- 

 round, the wave after transmission through the particle is retarded, or, 

 in other words, a small phase difference is introduced between the waves 

 S and P. This phase difference is represented as a slight displacement 

 of the two sine curves along the horizontal axis in the appropriate 

 direction. We may now determine just how the curve P differs from 

 the curve S by subtracting *S from P. It is found that the difference is 



