Page:Proceedings of the Royal Society of London Vol 60.djvu/181

166 F ig. 3.

D 'F + GrE' —2 a sin ^ nearly; Whence

IK-f-EGr — -f- gA. KL = — m— sin

In the case of the razor on which these measures were made N = 85 = 3 n2 = 2 aje = 8a —0-00405 in. sin = and since for soda light = 0-.00Q0116 in. nearly KL = 0-0000116 x 88-0-00405 x 0-25 = 0"00102—O'OOIOI, nearly.

Thus K L is not greater than 0*00001, and if it is assumed that the actual edge has the curved crOss-section, indicated by the dotted line in fig. 3, the radius of curvature cannot be greater than 1/200,000 of an inch.

A well sharpened razor will cut a hair, when merely pressed against it at about an eighth of an inch, or rather more, from the place where the hair is held. .

Human hair taken from the head has a circular cross-section, and varies in diameter in different individuals from 0‘002 to 0-004 in.

With a hair of 0-0025 in, diameter, fixed at one end and free at the other, it was found that half a grain acting at an eighth of an inch from the fixed end, bent it through an angle of about 30°.