138 ESSAYS BIOGRAPHICAL AND CHEMICAL 



at one side, not only was there the image of a circle 

 exactly opposite the hole, but also, at some distance from 

 the circular spot, a diffused drawn-out impression, as if 

 some of the rays had been unequally deviated by the 

 magnet, and had impressed the plate separately. It is 

 therefore probable that cathode rays contain some 

 X-rays. 



The wave-length of light can be measured by reflection 

 from a metal plate on which from 14,000 to 25,000 parallel 

 lines are ruled in each inch; such a prepared plate is 

 termed a ' grating ' ; the modern gratings, which are 

 wonderfully accurately ruled, are made by Mr. Brashier 

 of Alleghany, New York State, by means of apparatus 

 devised by the late Professor Rowland of Baltimore. 

 Careful measurements by M. Perrin have proved that if the 

 X-rays are due to ethereal vibrations at all, these cannot 

 possess a wave-length greater than 0*04 /JL, that is, less than 

 half the shortest- waved ultra-violet vibrations which have 

 ever been photographed. 



Again, when light is passed through a slice cut from 

 a crystal of tourmaline it is said to be polarised ; it can 

 pass through a second plate of tourmaline if held in a 

 particular position, but if the second plate be rotated so 

 that its second position is at right angles to its first, the 

 light is cut off, and fails to pass through the second plate. 

 M. Becquerel found that X-rays cannot be polarised ; they 

 pass easily through plates of tourmaline in whatever 

 position relatively to each other they be placed. On the 

 other hand, the rays emitted by phosphorescent bodies, 

 which may be termed the Becquerel rays, are capable of 

 polarisation. Hence they cannot be identical with X- or 

 with cathode rays. 



Lastly, it will be remembered that ultra-violet rays 

 discharge negatively electrified bodies; they are without 

 rapid action on bodies possessing a positive charge. But 



