70 PROGRESS IN MICROSCOPY 



through the image S[. This is where the effect on the direct hght can 

 be talcen without much alteration of the diffracted hght. A small trans- 

 parent plate is placed at S[ so that all the direct light passes through it, 

 the small portion of diffracted light passing through the plate being 

 disregarded. After selecting the proper thickness and index for the 

 plaie Q (Fig. 2.4), (called phase-plate), the direct vibrations (sinusoid V.y) 

 can be made to lead or lag without altering the diffracted vibrations 

 (sinusoid Kg). Figure 2.3 shows that the phase-plate is to lead or lag 

 the direct vibrations Kg by a quarter wave-length in relation to the 

 diffracted vibrations V^: the phase-plate is a quarter-wave-length plate. 

 The optical path of the direct vibrations traversing the phase-plate 

 of thickness e and of index A^ is denoted by Ne while the optical path 

 of the diffracted vibrations passing next to it, e.g. in air, is denoted 

 by e. If A is the wave-length of the hgh< used, then: 



Ne-e = {N-i)e - ^/4. (2.6) 



The direct vibrations are lagging and the arrangement is that 

 shown in Fig. 2.5(a): the phase-contrast is called negative phase-con- 

 trast. Let us increase the thickness c of the phase-plate so that: 



{N-{)e - 3A/4. (2.7) 



The lag of the direct vibration is now 3jr/2 = Itt—ttJI. As a Itz 

 lag does not change anything, phenomena occur as if the direct 

 vibration led by jijl, as in the arrangement shown in Fig. 2.5(b). Phase- 

 contrast is then positive. Assuming that // is larger than //', the 

 bacterium outshines the remainder of the field in negative phase- 

 contrast and is less bright in positive phase-contrast. The results are 

 tabulated hereunder. 



n = index of observed detail (bacterium) 



«' = index of medium encompassing the detail observed. 



