I9I3] 



BRUSH— SOME DIFFRACTION PHENOMENA. 



277 



power of 100 to 200 diameters. The focal plane of the objective is 

 usually adjusted near the diffracting edge as indicated by the dotted 

 line, and it nuist be borne in mind that this is where the fringes 

 are seen. 



! > 

 U 



IlT 



Fig. I. 



From E to F a series of cylindrical edges of progressively in- 

 creasing radii is indicated. E, however, is a sharp razor blade, and 

 the fringes projected by its edge are shown, greatly magnified, at K. 

 They are weak, few in number and hazy in outline ; but these con- 

 ditions are not due to any irregularity of the edge. When a fine 

 wire is used the fringes are distinctly better. Every time the radius 

 of the cylindrical edge is doubled, the fringes are unmistakably 

 brighter and sharper. L indicates the fringes produced by the cylin- 



