PHYSICAL ASPECTS OF IMAGE FORMATION 



61 



the same reflectance is obtained in both cases but the smallest diver- 

 gence from this wave-length causes the reflectance to differ widely. 

 In this way, stray light in microscopes was much curtailed. Intensity 



4. 



5. 



6/^ 



Fig. J. 78. Reflectance variations of a glass: (1) glass surface coated with one thin film; 

 (2) glass surface coated with two achromatic thin films. 



of the Stray light can be reduced almost tenfold and concurrently 

 improves the instrument considerably. 



12. REDUCING THE GRAIN OF PHOTOGRAPHIC IMAGES 



In some apphcations, the objectionable grain of photographic plate 

 is evidenced. A typical example is that of a microscope autoradiograph. 

 The granular structure of a photographic image can be reduced by 

 the foflowing process (Fig. 1.79). A fine radiant S in the focus of 



Fig. 1.79. Optical filtering of a photographic image. 



the objective C illuminates, with a parallel beam, the photographic 

 plate P requiring improvement. The objective O images the plate P 

 on to E. The imaged source S is at S' . This image is a diffraction 



