DELINEATION OF THE FINE STRUCTURE OF OBJECTS. 



227 



If A B (Fig. 124) is an object with alternately transparent and 

 opaque stria?, the figure representing a transparent part a I and the 

 adjoining opaque parts A a and b B\ 

 then, of the rays incident at right 

 angles on the left side, part only con- 

 tinue in the same straight line after 

 their passage through a b, another part 

 being deflected and forming divergent 

 luminous pencils which completely 

 occupy a given angular space. In the 

 figure one of these pencils is repre- 

 sented with a deviation of 30 to the JT IG 124. 



direction of the incident rays. Assum- 

 ing now the breadth of the transparent portion a b = 1 mic., and 

 the wave-length of the incident light (considered to be homo- 

 geneous) = *5 mic., then we get for the quantity a i the proportion 



a i : a b = sin 30 : r ; 

 sin 30 C 



therefore 



-. a b = *5 mic. = 1 wave-length. 



If we divide the deflected pencil into two equal portions, by 

 drawing from the middle of the aperture a line parallel to the two 

 marginal lines, it is at once evident that every ray of the one half, 

 as compared with the corresponding ray of the other half, will 

 appear to be displaced by half a wave-length. The combined 



action therefore of the 

 f ' two halves must neces- 

 sarily produce dark- 

 ness, if the rays are all 

 united by appropriate 

 ' refractions. If, for ex- 

 ample, we adjust the 

 objective of a Micro- 

 scope to a given object, 

 a dark line will ne- 

 cessarily appear in the 

 focal plane where the 

 rays of the deflected 

 pencil unite. It is the 

 first dark line which appears by the side of the bright line which 

 the incident rays produce in the focus of the objective. Let / <?, 



J?IG. 



