i6 



ON INTERFERENCE IN THE MICROSCOPE. 



the same. The bright interference lines (spectra of second order) 

 appear, therefore^ in those points for which the sine of diffraction - 



angle o-= -j , — j- , — r- , —r- &c., where A represents the wave 



length. Accordingly, where X=o.5 mik., calculation gives for the 

 relative values of a and d, the following figures : 



Consequently, when ^=\, a will under all circumstances =90, 



Fig. 4. 



From this it appears that with 

 central illumination no objective 

 can take up even the first image 

 forming diffraction pencils when 

 the mutual distances of the open- 

 ing are less than a whole wave 

 length (=0.^ mik.) 



These relations stand other- 

 wise when the direction of the 

 incident rays is oblique to the 

 axis. In such a case (see fig. 

 4) there is added to the differ- 

 ence of phase (J n) of the diffracted pencils, the difference 



