The razor applied at the same distance from the fixed end would sometimes cut through the hair before it had bent it as much as 30°; and this shows that a force of half a grain must make the pressure per unit area at the place of contact sufficient to cause crushing or disruption of the material even when the edge has entererd the hair to a distance comparable with the radius of the latter.
If we assume that the thickness of the edge is 1/100,000 in. and that it has entered the hair until the length of the edge engaged is 1/1,000 in., the area in contact will be able 1/100,000,000 of a square inch and the pressure per square inch rather more than 3 tons, if the total force over the area of contact is half a grain.
It is difficult to get any direct measure of the pressure required to destroy by crushing or shearing the material of which hair is composed, but horn which is of the same nature requires a much larger pressure than 3 tons per square inch to crush it.
A rough experiment shows that a cylindrical steel punch with a flat end, began to sink into a block of horn when the pressure was between 12 and 16 tons per square inch.
It would seem, therefore, that although the optical method shows that the thickness at the edge cannot be greater than 1/100,000 inch, the real thickness judged by the pressure per unit area necessary to cause the edge to cut in the way it usually does, must be considerably less than this.
While engaged in the determination of the "Indices of Refraction of various Substances for the Electric Ray" (vide 'Proceedings of the Royal Society,' vol. 59, p. 160), it seemed to me that the results obtained would be rendered more definite if the wave-length of the radiation could at the same time be specified. Assuming the relalation between the dielectric constant K and the index μ as indicated by Maxwell, to hold good in all cases, it would follow that the index could be deduced from the dielectric constant and vice versâ. The values of K found for the same substance by different observers are, however, found not to agree very well with each other. This may, to a certain extent, be due to the different rates of alternation of the field to which the dielectrics were subjected. It has been found in general that the value of K is higher for slower rates of alternation
