PHASE-CONTRAST MICROSCOPY 69 



the H 7' summation of the constituent-vibration amplitudes: The 

 light intensity h in the imaged bacterium A'^ then becomes: 



/i = (l + 9^)'^l+2<p (2.4) 



since 7- is disregarded. Next to the bacterium, the intensity A remains 

 equal to 1. The intensity ly in the imaged bacterium differs from the 

 intensity I2 in the remainder of the field : the bacterium becomes visible. 



(a) (b) 



Fig. 2.5. By introducing the phase plate, sinusoid K and K3 arc in phase (a) or in 



opposition (b). 



In Fig. 2.5(b), the vibrations K, and Kg are in opposition and are 

 subrtacied from one another. The resulting amplitude in the imaged 

 bacterium is \~(p and the intensity: 



h-^{l-qf^i-2q. (2.5) 



/i differs from /.: the bacterium is still visible. 



In the first case (Fig. 2.5(a)), A is larger than L. The bacterium 

 outshines the remainder of the field. In the second case (Fig. 2.5(b)), 

 /i is smaller than h and the bacterium is not as bright as the remainder 

 of the field. Thus shifting the curve V^ by njl in relation to its initial 

 position, i.e. dephasing the direct vibrations by njl in relation to the 

 diffracted ones, enables one to convert the small phase differences in 

 the object (index or thickness differences) in fight-intensity differences 

 in the image. This is the basic principle of phase-contrast microscopy. 



We have just seen now phase-shifting of direct and the diffracted 

 light by Tijl allows observation of a specimen consisting of smaU 

 transparent details. How can the motion of sinusoid K3 be achieved? 

 Reference to Figs. 2.1 and 2.4 shows that this problem can be solved 

 in a simple manner. We have seen that the direct illuminating fight 

 and the bacterium-diffracted light are severed in the plane F passing 



