PHYSICAL ASPECTS OF IMAGE FORMATION 



41 



evolves in accordance with the aperture of the condenser. Ordinates 

 show the hght intensities taking as unit the intensity at a point far 

 removed from the imaged black disk; abscissae show the distances to 

 the imaged geometrical centre. The geometrical width of the imaged 

 black disk is shown by the distance included between the two small 

 vertical hnes above the curves in Fig. 1.52. When Dy is almost fully 



Fig. 1.52. Image of a small black disk in relation to the aperture condenser: (1) coherent 

 illumination; (2) partially coherent illumination; (3) incoherent illumination. 



diaphragmed down (coherent illumination) curve I shows the result. 

 When the diaphragm aperture is increased (partially coherent illumi- 

 nation) so as to impart to the condenser a N.A. equivalent to that 

 of the objective, curve 2 shows the result. When illumination is 

 incoherent, curve 3 shows the result. 



These curves enable one to state the following: as the condenser 

 N.A. is increased, the clearer the image, the less prominent the 

 diffraction fringes, without this virtually entailing a change in contrast 

 of the image. Moreover, in order to prevent stray light, the condenser 

 has a N.A. lower than that of the objective. Therefore it can be said 

 that the image contrast of a small object, in microscopy, always 

 equates the image contrast in coherent illumination. Since diaphragm- 

 ing down D^ leads to extending the image to the detriment of its 

 quality, it follows that it is best to open suitably the diaphragm D^, 



