INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 97 



To sum up, there are the following vibrations: 



(a) The vibration (3), defining the luminous amplitude next to 

 the object; 



(b) The vibration (3) and (5) which, combined, detine the luminous 

 amplitude in the object. 



The phenomenon is similar to that of phase-contrast. The amplitude 

 in the imaged object equates the summed amplitudes of the vibra- 

 tions (3) and (5). The amplitude of the vibration (3) is adjustable at 

 will: if (2) exactly in opposition to (I) the ground is dark as (3) is zero, 

 but the required amplitude of the vibration (3) can be obtained by 

 altering slightly, by means of the interferometer, the phase difference 

 between (2) and (1). Phenomena occur as if a phase-plate, adjustable 

 for phase-displacement and absorbence, were used. Up to now, we 

 did not give any clues as to interferometer design, i.e. how dissociation 

 and recombination of the light-rays occur at M and A^ (Fig. 3.1). 



There are many types of such instruments amongst which are those 

 of Dyson, Krug and Lau (cf. Chapter IV), Philpott, Nomarski. 



3. DYSON'S INTERFERENCE MICROSCOPE 



Figure 3.4 is a diagram of Dyson's microscope. Let us consider 

 the light-ray S from the condenser (not shown in Fig. 3.4). This ray 

 passes through the parallel-sided glass plate L^ whose upper face is 

 semi-metallized and on whose surface the ray branches off in two 

 sections: one towards the incident ray, the other reflected at M 

 towards the instrument's lower part. The former ray passes first 

 through the specimen P at^i where the object lies and also through the 

 second plate L^ of same thickness as Li, whose faces x^x'^ and x-iX'^ are 

 semi-metallized. It is reflected at C on the upper face of Lo, then at N 

 on the lower face and along the path NEF. The latter ray reflected 

 at M towards the instrument's lower part is now reflected on to the 

 lower part of L^ at B. In this area, the plate L^ is coated with a small, 

 highly-reflective opaque-silvered disk. After B, the light-ray travels 

 along BN, parallel to MC. The two rays MACN and MBN rejoin 

 at N and merge with the NEF ray. Figure 3.5 shows separately how 

 the foregoing paths tally with the basic diagram (Fig. 3.1). It follows 

 that the object is not illuminated solely by the light-ray just described 

 but by an infinity of such rays, bunched cone-hke from the condenser. 

 Figure 3.4 shows the path of a ray symmetrical to the foregoing ray 

 in relation to the system's axis. Since the specimen P is between the 



