Note of the Radius of Curvature of a Cutting Edge. 165 



np to the edge of the blade, gives the distance BC in terms of the 

 half wave-length. 



The distance, BD, was measured by the micrometer, and the 

 number obtained by dividing BD by the distance between the con- 

 secutive bands in the neighbourhood of the edge (since the thin 

 glasses are hardly at all bent so near their free ends) gives when 

 added to N the number of half wave-lengths in DE. 



After these measures had been made, a piece of flat glass was laid 

 on the blade of the razor as in fig. (2) and the number of interference 



FIG. 2. 



bands which appeared between the edge and a line parallel to the 

 edge, but distant BD from it, was counted; and this observation was 

 repeated with the flat glass on the opposite side of the blade. 



The angle H P I, i.e., the angle between the two positions of the 

 flat glass, was also measured. 



If the grinding of the razor was perfect and there was no rounding 

 at the edge, no interference bands would be visible between the 

 blade and the glass, but the two would be in contact up to the 

 actual edge. 



Fig. (3) is a large scale cross-section of the blade in the neigh- 

 bourhood of the edge. 



The thickness of the edge K L is 



If we put N = number of bands between the clip A and the razor 

 edge ; 



e = the distance between consecutive bands near the edge ; 



1 > = the number of bands between the flat glass and either 

 side of the blade in a distance DB from the edge ; 



BD = a and HPI = 0. 



