26 



PROGRESS IN MICROSCOPY 



occur as if source Si was the only one although the phenomenon seen 

 on the screen is naturally brighter. 



A and B, therefore, can be illuminated with a large source in the 

 plane D^ without altering the visibility of the fringes, provided that 

 the angle 6, at which the source is seen from A and B, be small 

 {6 must be small with respect to the interval between two fringes as 

 seen from A or B). When such is the case, from the standpoint of 

 image contrast phenomena occur as if apertures A and B diffract 

 coherent vibrations. 



If the angle is increased, staggering of the various fringe systems 

 due to the various elements of the source becomes prominent; phe- 

 nomena do not coincide any longer, are blurred to a lesser or greater 

 degree and contrast is decreased. If the angle is increased still 

 further, a point is reached when phenomena are completely blurred, 

 contrast is zero and fringes vanish. The two apertures A and B act as 

 two incoherent sources and, of course, no interference can take place. 



Let us now consider the next experiment (Fig. 1.29). The pin-point 

 source S^ is imaged on the plane P by means of the lens C. Assuming, 



■"0^ 



■^ 



rl 



■^ 



\ 



•^B 



Fig. 1.29. Points A and B arc illuminated in coherent light. 



first, that aperture a of the lens C be very small and, hence, the image 

 of So a well spread-out diffraction disk. Let points A and B (in plane P) 

 be close enough to be within the central diffraction disk. Under such 

 conditions, both theory and experimentation show that points A and B 

 are illuminated in coherent light. Were A and B to consist of two holes 

 pierced in the plane P, they would produce interference fringes on 

 screen E as in the previous experiment (Young's apertures). Let us 

 increase the angle a, viz, aperture of the lens C The diffraction disk 

 shrinks more and more and no alteration is evidenced if the distance 

 from A io B ']s reduced adequately. The range within which A and B 



