48 



PROGRESS IN MICROSCOPY 



penetrate into the objective, the image looks more and more like 

 the object. 



Let us consider a grating whose slits are very close and let us 

 operate in obhque illumination. The layout may be that shown in 

 Fig. 1.62 where only the direct image 5,', and the spectrum S!, penetrate 

 into the objective. The plane P' is illuminated by the two coherent 

 sources Sq and S!,. The imaged graiing is a sinusoidal image similar 



Fig. 1.62. Periodic object in oblique illumination. 



to that in Fig. 1.48 (Young's fringes produced by the two sources 

 S'l and S!, in the plane P'). Therefore, all that is needed to see the 

 imaged grating is that the objective N.A. be such that the image 

 Sq plus a spectrum, e.g. So, may penetrate into the objective. Oblique 

 illumination of the grating shows a sinusoidal image the period p of 

 which is given by 



P=:r^- ^'-^^ 



2/7 sin M 



The expression shows the resolving power of a Foucauit type 

 periodic object in coherent illumination. The numerical value obtained 

 is the same as in incoherent illumination (expression 1.6) although the 

 resolving power is then reached after a steady dwindling of the image 

 contrast. Jn coherent illumination, the process is different: as long 

 as two images at least penetrate into the microscope's objective the 

 periodic structure is clearly seen. The imaged grating vanishes only 

 when one of the two images 5^ or S^ is arrested by the objective. 



10. PERCEPTION LIMIT AND RESOLVING POWER 



The foregoing results are collected and discussed further in the 

 present section. 



If the phenomena are considered in the plane of the specimen, 

 a pin-point detail can be substituted for a small disk of radius 



