PHYSICAL ASPECTS OF IMAGE FORMATION 



31 



the specimen (Fig. 1.31). Areas not within the field of view are un- 

 illuminated and spurious light is rninimized. Lastly a corrected con- 

 denser (aplanatic) provides the evenly illuminated field, indispensable 

 in photomicrography. 



To sum up it can be said that for objects of high contrast and 

 smaller than the central diffraction disk of the objective, images are 

 virtually in coherent illumination. Such is the case of the more 

 valuable details observed in a microscope. 



Naturally, coherence varies somewhat when the condenser aperture 

 is altered and, even when close to small-object coherent illumination, 

 differences in appearance are developed whose effects are far-reaching 

 and will be discussed later. 



For objects of low contrast the phenoma are not the same. Coher- 

 ence varies much more quickly. For a small black disk, illumination 

 is again virtually coherent when apertures of the objective and con- 

 denser are equate. Under the same conditions, illumination is incoher- 

 ent for an object of low contrast. 



In general, incoherent illumination does not come in microscopy 

 except for the object of low contrast and in fluorescence. In the latter 

 case the specimen is illuminated by ultraviolet light and emits visible 

 light. The various points of the specimen are then incoherent. 



8. EXTENDED OBJECTS IMAGED IN INCOHERENT ILLUMINATION 



When considering the extended object P (Fig. 1.36), it may be 

 broken up into small luminous elements acting as discrete pin-point 

 sources. Thus the object P can be considered as consisting of minute 



Fig. 1.36. Image formation of an extended object. 



luminous sources A, B, C, etc. that can be likened to real pin-point 

 sources. The object P is seen in incoherent illumination provided its 

 constituent sources A, B, C act as thoroughly independent sources 



