38 



PROGRESS IN MICROSCOPY 



of the bright hnes and Z, that of the dark ones. As with the previous 

 objects, the contrast can be determined by the equation (1.3). In the 

 image plane, intensity distribution has the shape shown by curve 2 

 in Fig. 1.47. Curve 1 shows the geometrical image. Therefore, the 

 phenomenon obtained is always the same: there are no breaks; out- 

 lines are smoother and contrast is decreased. 



41 



r\ /IN 



(I) 



i2) 



Fig. 1.47. Foucault test image — (1) geometrical; (2) actual image. 



Theory shows that a Foucault test of the type depicted in Fig. 1.45 

 can be shown by superposing any number of sinusoids, similar to 

 those in Fig. 1.48, the periods of which are p, \p, \p, etc. It follows 

 that the instrument transmits the components of long periods (spaced- 

 out lines) much better than those of short periods (Hnes close together). 

 Applying the term "frequency" to the converse of period y;, it can be 

 said that an instrument always conveys the low-frequency components 



Fig. 1.48. Sinusoid tiie period of which is p. 



better than the high-frequency ones, which is tantamount to saying 

 that the closer together the elements of a periodic structure the less 

 resolvable they are. Educing how such frequencies are transmitted by 

 a specific instrument is desirable. The curve (Fig. 1.49) shows the 

 results. Frequencies 1//? are plotted as abscissae while the contrast 

 factor, the parameter A/, which determines how every frequency is 

 transmitted by the instrument, is plotted as ordinates. If M = 1, 

 the frequency considered is transmitted faultlessly; if M = 0, it ceases. 

 At very low frequencies, the contrast factor is virtually equivalent to 1 

 and, hence, they are well transmitted. As the frequency rises, viz. when/? 



