58 The History and Properties of the Rontgen Rays. 



of the various parts corresponding to the differences of opacity 

 of the various parts of the object. (5) A curious effect of X rays 

 was their power of discharging electrified bodies. When a 

 gold leaf electroscope was charged, and the X rays fell upon it, 

 immediately the leaves collapsed. (6) X rays suffered very 

 little reflection or refraction, and thus they could not bring 

 Ihem to a point by mirrors or lenses. (7) Unlike cathode rays 

 or Lenard rays they were not deflected by a magnet. Turning 

 next to the question of theory, he wished to say, first, a few 

 words about light. They knew that light consisted of transverse 

 vibrations in the ether, the rate of vibration varying with the 

 colour ; thus violet rays had about twice as many vibrations 

 per second as red rays. Now, besides those visible rays there 

 were rays of the same intrinsic nature, some of fewer vibrations, 

 the infra red, which had been carefully measured by Langly in 

 America ; others of a greater number of vibrations per second, 

 the ultra violet, which had been investigated by Schuman in 

 Austria. Taking the analogy of sound, and remembering that 

 when one note had just twice as many vibrations as another, 

 they called it an octave of that note. They could easily imagine 

 a scale of light consisting of six octaves, like the notes on a 

 piano — three octaves of infra red ; then in the centre the 

 seven colours : red, orange, yellow, green, blue, indigo, violet, 

 and next two octaves of ultra violet. Now, violet and ultra 

 violet rays falling on a fluorescent substance made it luminous — 

 indeed, this was how ultra violet rays were discovered. X rays 

 did the same. Ultra violet rays discharge a negatively electri- 

 fied body. They had seen the effect of X rays. From these 

 analogies they might say that there was a strong presumption 

 that X rays are of the same intrinsic nature as ordinary light. 

 On the other hand, the absence of reflection and refraction in 

 X rays seemed to go against this. Now, Helmholtz, a few 

 months before his death, a good while before the discovery of 

 X rays, published a paper in which he proved that for extremely 

 rapid vibrations there would be no refraction. Various con- 

 siderations lead them to think that X rays were of this nature ; 



