258 Pto£. C. G. Barkla : Experiments to 



Experiments had previously been made on the same 

 subject by a number of investigators, but they had not been 

 of such a kind as with our present state of knowledge to 

 justify any hope of success. 



The refracting surfaces were usually ordinary optical sur- 

 faces, in which the irregularities were in general large 

 compared with the wave-length of Rontgen radiation. This 

 being the case, any refraction occurring would be irregular 

 refraction which would produce only a certain amount of 

 diffusion in a transmitted pencil and consequent blurring of 

 the " image. " The methods of detection, too, were not the 

 most sensitive. 



In the following experiments a prism of large angle and 

 with crystalline refracting surface was used ; the pencil of 

 radiation transmitted was an exceedingly narrow one ; the 

 experimental arrangement was such as to make a small 

 deviation measurable ; and the material of the refracting 

 prism and the transmitted radiation in certain cases were 

 such that the frequency of the latter was not far removed 

 from the natural frequency of vibration of one set of electrons 

 in the other. 



The refracting substances used in these experiments were 

 rock-salt and potassium bromide, the two refracting surfaces 

 being crystalline planes at right angles. A narrow pencil of 

 X- radiation was passed through the refracting right-angled 

 prism and was received on a photographic plate on the 

 other side. 



In the initial experiments the X-racliation was sent through 

 two parallel slits in parallel lead screens. In the path of 

 this pencil beyond the second slit was placed the prism with 

 its refracting edge parallel to the slits but intercepting only 

 one-half of the beam. Thus, of the very narrow pencil, one- 

 half * passed through the air just outside the prism, while 

 the other passed through the prism. At first curious effects 

 were produced on the photographic plate beyond, some of 

 which were suggestive of refraction : the two portions of the 

 beam were apparently separated, for a clear line appeared 

 between the two dark "images" on the negative. It was, how- 

 ever, found that such effects, and fringe effects observed at 

 the edge of a shadow, were due to (1) the finite magnitude 

 of the source, and (2) the lack of proportionality between the 

 density of a photographic image and the intensity of the 

 radiation producing it. 



These results led to nothing fundamentally new; they are 



* Of half the width. 



