GENERAL CONSIDERATIONS OF DESIGN 77 



practice an extended opening in the condenser diaphragm and hence an 

 extended source of Hglit is used to iUuminate the specimen, it is not 

 possible with most object specimens to separate completely the deviated 

 from the undeviated light. The use of a white light source rather than 

 a monochromatic source will increase the amount of overlap of the 

 deviated and imdeviated light at the conjugate area. As the range of 

 numerical aperture included in the opening of the condenser diaphragm 

 becomes smaller (i.e., as the opening becomes narrower) the conjugate 

 area of the diffraction plate becomes narrower and there is less overlap 

 of the deviated and undeviated light at the conjugate area. This is the 

 most important reason for making the conjugate area narrow. 



It is known that loss of resolution occurs when the numerical aperture 

 of the condenser is reduced in value below that of the objective. The 

 resolving power of a phase microscope is discussed to some extent in 

 Section 18 of Chapter II. If the specimen is illuminated by a hollow 

 cone of light, as when the condenser diaphragm contains an annular 

 aperture, theory shows that resolution can be expected to increase as 

 the outer diameter of the illuminating cone is increased (greater p2. 

 Fig. 11.20). If the numerical aperture corresponding to the outer di- 

 ameter of the illuminating cone is sufficiently great, then some further 

 advantage may be gained by also increasing the inner diameter of the 

 illuminating cone (smaller [po — Pi], Fig. 11.20). The relative trans- 

 mission of the conjugate and complementary areas and the optical 

 path difference between these areas will have some additional effect on 

 resolution. How^ever, this effect will be of secondary importance, at 

 least when the optical path difference between the particle and its sur- 

 round is small. Therefore, in order to gain the improvement in contrast 

 which can result from the choice of relatively narrow conjugate areas and 

 in order to maintain as high resolution as possible, an annular opening 

 in the condenser diaphragm is generally preferred to a circular aperture 

 centered on the optical axis. 



It is also a fact that an extremely narrow opening in the condenser 

 diaphragm and the correspondingly very narrow conjugate area super- 

 impose undesirable diffraction effects on the image of some specimens. 

 For some purposes, notably when photomicrographic records that 

 require short exposure times are to be made, a very small condenser 

 opening may cause too great a loss in illumination. A useful rule of 

 thumb is to make the width of the conjugate area (the zone included 

 between di and 62, Fig. 11.20) approximately equal to one-fifteenth of the 

 clear aperture of the objective in the plane of the diffraction plate 

 (Zernike, 1942). 



When existing condenser and objective systems are used in the design 



