568 Elastic Constants of Glass. 



beams whose width is great compared with their thickness, 

 a tendency towards turning- up at the edges is shown, and 

 this may very well indicate a trace of the " blade " type of 

 bending. 



The condition usually accepted (c/. Searle's 'Experimental 

 Elasticity ') that the " beam" type of bending shall occur is 



7.2 



that the ratio =— of the beam shall be small compared with 



za 



the radius of longitudinal curvature. It is not clear, how- 



7 2 



ever, that a value approaching 50 for the ratio — would be 



sufficiently small to give uniform cross-bending with a radius 

 of longitudinal curvature of about 1000. It is intended to 

 investigate this fully with optically perfect glass. 



Summary of Results and Conclusions. 



Table VIII. gives a summary of the results obtained in 

 eight experiments. 



The evidence of an alteration with time in the apparent 

 value of Young's Modulus is conclusive. There is also in 

 most cases a marked change in the value of Roisson's Ratio 

 as determined by this method, but whether this indicates a 

 change in the actual value of the ratio, or whether it is due 

 entirely to a change in the shape of the transverse curvature 

 is not certain. The method does not admit of an accurate 

 determination of the curvature of the beam near the centre 

 owing to the distance apart of the fringes in that region, 

 and it is possible that a change of curvature in the centre 

 portion of the beam might counterbalance to some extent 

 changes in the outer portions over which the observations 

 were taken. 



In conclusion the author desires to express his acknowledg- 

 ments to the Department of Scientific and Industrial Research 

 for their assistance which enabled this work to be carried 

 out. 



