164 Mr. A. Mallock. 



point of silver ; and the same is true of silver-platinum. The silver 

 aluminium curve presents some singularities ; but here, as with other 

 aluminium alloys, we have been troubled by partial oxidation of 

 the aluminium, and we therefore hope to revise our experiments 

 with this metal, before publishing them in full. 



" Note of the Radius of Curvature of a Cutting Edge." By 

 A. MALLOCK. Communicated by LORD KELVIN, F.R.S. 

 Received June 9, Read June 18, 1896. 



The following note may be of interest, partly as indicating the 

 extreme thinness to which a cutting edge may be brought by the 

 ordinary process of grinding, and partly also as showing how readily 

 bhe wave-length of light may be used, with only the simplest appli- 

 ances, as a practical unit for the measurement of small distances. 



The object in view was to find the thickness, or at any rate a 

 superior limit to the thickness, of the cutting edge of a razor, and for 

 this purpose two pieces of thin glass (such as is used for covering 

 microscope slides) were prepared about J inch long and -^ wide. 



These were pressed together by a small steel clip A, and the edge- 

 of the razor was inserted between them as shown in fig. (1). 



FIG. 1. 



The razor with the thin glasses in this position was then placed on 

 the micrometer stage of a microscope and illuminated perpendicularly 

 with light from a soda flame. 



With the microscope, interference bands were of course visible 

 between the thin glasses; and the number of bands, (N), counting 

 from the spot where the clip pressed the glasses into optical contact 



