76 PROGRESS IN MICROSCOPY 



is given by expression 2,9. It can be considered that limit of perception 

 is attained when its contrast attains 004. Then equation 2.9 gives 



y = 2(f] N = 004. (2.12) 



As the phase-plates generally used do not exceed N = 100, then: 



in-n')e =0 0002. (2.13) 



An object of thickness e = 1 // could only be seen, for instance, 

 provided that the index differences between it and the medium sur- 

 rounding it exceeds 2xl0^\ Stray light impairs sensitivity and its 

 effects are the greater as phase-plate absorption increases. If intensity 

 of the stray light is a fraction K of the incident intensity, the image 

 phase-contrast is divided by \ \ NK. When A^ = 100 and K = 005 

 the image contrast is divided by 6. While the phase-contrast method 

 does enable one to perceive details otherwise remaining invisible under 

 normal conditions, its very principle is not conducive to improved 

 resolving power. General diffraction laws, i.e. the structure of the 

 diffraction pattern displayed by the objective, determine the resolving 

 power of a microscope. The presence of the phase-plate alters but 

 slightly the diffraction pattern. The phase-plate occludes a small 

 portion of the central aperture of the microscope objective, thus 

 slightly altering the diffraction pattern: the central disk is narrowed, 

 thereby increasing the hght in the diffraction rings surrounding it. 

 The resuh is that ihe resolving power in the case of high-contrast imaged 

 objects is slightly improved. Conversely, low-contrast imaged objects 

 afford less improved resolution. A phase-contrast microscope provides 

 approximately the same resolving power as an ordinary microscope. 

 When observing a transparent object immersed in a medium the index 

 of which is very similar to it, the details are not perceivable in an 

 ordinary microscope. But, once the phase-contrast microscope has 

 converted the phase changes into intensity variations, two discrete 

 particles are no farther apart when resolving power is reached than 

 with an ordinary microscope when observing a non-transparent object, 

 featuring light-intensity variations. 



6. PHASE-CONTRAST MICROSCOPES IN TRANSMITTED LIGHT 



Referring to Fig. 2.1, all the illuminating direct light passes through 

 the aperture Si of the diaphragm D^, located in the focal plane of 

 condenser C. After passing through the condenser C and the objec- 



