PHYSICAL ASPECTS OF IMAGE FORMATION 



39 



drops, so does the transmission of the sinusoidal object. The trans- 

 mission becomes zero when 1//? =^ (2/7sinw)/A (the period p being 

 measured in the object plane). The instrument does not transmit 



Fig. 1.49. Contrast factor of an optical instrument. 



frequencies exceeding (2//sinz/)/A; the object-rod can therefore be dis- 

 tinguished only if its bars are spaced by /?, as follows: 



P 



Insmu 



(1.6) 



The equation (1.6) is tantamount to showing the instrument's 

 resolving power. Therefore, with a test of the type shown in Fig. 1.45, 

 the period of which is p, when resolving power is reached, the imaged 

 test is a sinusoidal image exhibiting the same period whose value is 

 derived from the above equation. For instance, with a 130 N. A. 

 objective and A = 06// a periodic structure p of at least 023 /^ can 

 be perceived in incoherent illumination. 



9. EXTENDED OBJECTS IMAGED IN COHERENT AND PARTIALLY 



COHERENT ILLUMINATION 



Referring to Kohler illumination (Fig. 1.50), let us diaphragm 

 down almost completely the condenser lens D^^ . The light source from 

 the very small aperture 5^ forms a cylindrical pencil of parallel rays 

 that passes through the specimen P and penetrates into the ob- 

 jective Oi. Owing to the direct light from S^ a small detail A diffracts 

 some of it. The wider the angle 0, at which the light is diffracted hy A, 

 the smaller the detail considered. The hght diffracted by A is collected 

 by the objective O^ and converges on A'^. The image A'q of the 

 detail A results from superposing the coherent ground B'C and the 



