56 



PROGRESS IN MICROSCOPY 



Assuming the edge S.^ to be at such a distance from the axis that 

 only the very edge S2 of the spectrum S^ penetrates into the ob- 

 jective Oi. The lower edge of S'q (lower edge of the spectrum S^) 

 conveys vibrations to the image plane P' which may bring about 



(a) 



(b) 

 Fig. 1.73. PIciirosigDia aiigtilatum; pleurosigiiia aiigiilaliini not rost»lvcd. 



interference; likewise, the upper edge of 5,', and S[ (upper edge o{ the 

 spectrum S[). The plane P' is illuminated by several sources which 

 interfere and an image of the grating is perceived. 



When the condenser diaphragm is almost fully diaphragmed, 

 neither the spectrum S[ nor S'. can penetrate into the objective. As 

 the diaphragm aperture increases the spectra S[ and SI spread out 

 and a time comes when a small portion thereof penetrates into the 

 objective, whereupon an image of the grating appears. Let us now 



