Cornu s Method of finding Elastic Constants of Glass. 551 



As is well known, this curve gradually flattens out at the 

 «nd of the range. It is shown that this characteristic shape 

 of the curve at the extreme end can be accounted for by small 

 variations in the ranges of the individual a particles. These 

 variations are due to probability variations in the number 

 of electrons encountered by the a, particle along its 

 path. 



It is pointed out that the usual definition of the range 

 is largely meaningless, and an " extrapolated range 5 ' is sug- 

 gested which is more definite and would seem to have 

 more theoretical significance. The values of this extra- 

 polated range in air at 0°0 are for R-iC, 6*592 cm., for ThCi, 

 4*529 cm., and for ThC 2 , 8' 16 7 cm. Finally, a general 

 equation for the end portion of the ionization curve is 

 suggested. 



It is a pleasure to tender to Professor Sir E. Rutherford, 

 who suggested the problem, my best thanks for his kind 

 advice throughout the course of this investigation. I also 

 wish to thank Mr. G. R. Crowe for the preparation of the 

 radioactive sources. 



LXIV. On Cornu' s Method of Determining the Elastic 

 Constants of Glass. By H. T. Jessop, B.Sc* 



[Plates XVIII. & XIX.] 



History. 



(1) IN a paper published in 1869 f, Cornu gave an account 



JL of a method of determining the elastic constants of 



glass by observations of the deformation of the surface of a 



rectangular glass beam subjected to a bending-moment. 



The surface of the beam acquires an anticlastic curvature 



which was measured by Cornu by means of interference 

 fringes produced between the anticlastic surface and the 

 surface of a plane cover-glass resting on the beam. The 

 interference pattern produced consists of two conjugate 

 systems of hvperbolas as shown in the photographs in fio-s. 

 2 and 3 (Fh. XVIII. & XIX.). 



Cornu produced the curvature by supporting the beam on 



* Communicated by Professor L. X. G. Filon, F.R.S. 

 t M. A. Cornu, Compt. rend. lxix. p. 333 (1869). 



