detect Refraction of X-rays. 



259 



only worth mentioning as of a kind likely to lead to wrong 

 interpretation. 



The final experiments are, however, probably worth record- 

 ing. The radiation from an X-ray tube passed successively 

 through two narrow vertical slits in parallel lead screens 

 about 50 centimetres apart, on to a photographic plate at a 

 distance o£ about 150 centimetres from the second screen. 

 The photographic impression produced by the pencil of 

 radiation was then a thin vertical line 3 centimetres long and 

 about '01 centimetre in width. A few centimetres away from 

 the second lead screen were placed two crystals of potassium 

 bromide arranged as two refracting prisms end to end— -that 

 is, one above the other — with refracting edges vertical but 

 with their bases on opposite sides of the transmitted radiation 



(see figure). Thus any refraction of the radiation in trans- 

 mission through the prisms would result in displacements in 

 opposite directions of the upper and lower halves of the thin 

 pencil of radiation. An exposure of a rapid plate was made 

 for a period of from five to eight hours. It was found that 

 the photographic impression was still a straight line from top 

 to bottom. There was no evidence of the slightest break or 

 lateral separation between the two halves of the image. 

 When looking at the line image in a direction making a 

 small angle with its length, it was estimated that a relative, 

 lateral displacement of '025 millimetre could have been 

 detected. 



T2 



