346 Messrs. Edser and Senior on the Diffraction of 



shown in figs. 12 and 13. The course of the curves for 

 large tension is, however, quite different from that for small 



Tig. 13. 



20x10 































10 











T = 







= 4*5 

















£ 



-— — 



— — - •- 



— - 





= 1299 



















h 



— — — 



"~ 





T 



= 24-52 







. 













l>^ 



IC 







'57 









%&' 







tension. For a tension of 4'76 kilograms per square milli- 

 metre the wire first contracts and then elongates when the 

 field is increased. The curve of the change of length is 

 therefore similar to that for cobalt. From fig. 12 we see 

 that the rate of diminution decreases as the tension is 

 increased. 



In these two sorts of nickel- steels the curves for T = are 

 the results of measurement obtained by Prof. Nagaoka and 

 one of us, and reproduced here for the sake of comparison. 

 It was at first our intention to perforin the same experiment 

 on a cobalt wire ; but having at present no such material 

 at our disposal, we leave the subject for future consideration. 



In conclusion, we wish to express our best thanks to 

 Prof. H. Nagaoka and also to Prof. A. Tanakadate for 

 useful advice and kind guidance. 



XXXVIIL The Diffraction of Light from a Dense to a Barer 

 Medium, wlien the Angle of Incidence exceeds its Critical 

 Value. By Edwin Edser, A.R. G.S., Lecturer on Physics, 

 Woolwich Polytechnic ; and Edgar Senior, Lecturer on 

 Photography, Woolwich Polytechnic *. 



nPHE well-known law of refraction 



sin i=fi' sin r, 



indicates that, for a refracted ray to be formed, the angle of 

 incidence, i, must be less than a certain critical value given 

 by the equation 



sin i = fjb'. 



* Communicated by the Authors. 



