Page:Proceedings of the Royal Society of London Vol 60.djvu/179

164 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.

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 the wave-length of light may be used, with only the simplest appliances, 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 ^ 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).

F ig. 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, interfei’ence 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