PHYSICAL ASPECTS OF IMAGE FORMATION 63 



featured by degrees in blackness, i.e. changes in contrast. In this 

 image the grain is evinced as, for instance, in Fig. 1.80. The contrast 

 changes in plate P are details needing investigation. Assuming, as is 

 the case in Fig. 1.80, that these details are much wider than the grains 

 to be eliminated: since the grains are minute and virtually of the same 

 diameter, they exhibit at S' an extensively spread diffraction pheno- 

 menon. Conversely, the photographic details to be investigated at S' 

 are much narrower than the grain's. Setting in the plane S' a screen 

 pierced with a hole of suitable diameter (determined by experience) 

 the major part of the grain-originated diffraction phenomenon can be 

 occluded. The narrow diffraction phenomena originated by the photo- 

 graphic details can pass through and a new plate is ultimately obtained 

 at E, where grain effects are substantially reduced. The screen, located 

 at S' and pierced with a hole, "filters"' the photographic plate P. 

 Figure 1.81 shows the photograph in Fig. 1.80 after such "filtering". 

 The experiment requires much care in order to prevent interpretation 

 errors. 



