On the Determination of Wave-length of Electric Radiation. 167 



The razor applied at the same distance from the fixed end would 

 sometimes cnt 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 entered 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 about 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 com- 

 posed, 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 showed 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 actually does, must be considerably 

 less than this. 



" On the Determination of the Wave-length of Electric Radia- 

 tion by Diffraction Grating." By JAGADIS CHUNDER BOSE, 

 M.A. (Cantab.), D.Sc. (Lond.), Professor of Physical 

 Science, Presidency College, Calcutta. Communicated by 

 LORD RAYLEIGH, Sec. R.S. Received June 2, Read June 

 18, 1896. 



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 rela- 

 lation between the dielectric constant K and the index JJL 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 versa. 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 



