206 Dr. A. A. Robb : Graphical Solution of Differential 



durability of rock-salt depends not on the surface layer, as 

 "W. Voigt supposed, but only on the direction of the slide. 

 Further " reversive " slides are observed at quite a definite 

 amount of strain. 



II. The Elastic Strain. 



If we take two Laue rontgenograms on -the same plate, 

 the first from a plain crystal and the second from the same 

 crystal under strain, we obtain double spots, and from the 

 distances between those spots we can measure the angles 

 through which different crystallographic faces rotate when 

 strain is produced. 



In order to measure the variation in the intra-atomic 

 distances, we use a diverging beam of monochromatic 

 Rontgen rays. The doable picture produced from the 

 normal and from the strained crystal shows in such a case 

 the variation of the grating constant. The monochromatic 

 rays were produced by a tube with a silver or a molybdenum 

 anticathode with a filter of 0*006 mm. thickness of palladium 

 or niobium respectively in the path of the rays. Under these 

 conditions and with voltages less than 40 k.v. the K a 

 radiation contains about 98 per cent, of the energy of the 

 beam. 



The method we use has the advantage that on two photo- 

 graphs strains in every direction are obtained and all the 

 constants of elasticity determined on one small sample of 

 the crystal. 



Roentgenological and Radiological 



Institute, Petrograd. 



1919. 



XIX. On a GrapJtical Solution of a Class of Differential 

 Equations occurring in Wireless Telegraphy. By Alfred 

 A. Robb, Sc.D., F.R.S.; with Note by E. V. Appleton, 



M.A* 



Part I. 



'FflHE differential equations which we propose to consider 

 JL are of the general form 



S+/(^ + »^=o. • • . (i) 



* Communicated by the Authors. 



