284 CATHODE RAYS AND ll()NTGEN RAYS. 



At the outset the mo.st surprising characteristic of these ra_ys is 

 their power of penetration. They pass as easily through a volume of 

 a thousand pages as a ray of light passes through a window pane. 

 Both cases exhibit the same prowess of nature; and if the latter fact 

 no longer astonishes us, it is because, as Montaigne says, "familiarity 

 with things removes from them their strangeness." Our surprise 

 arises in observing the newcomer accomplish that which was impossi- 

 ble for our old friend, light. We were formerly no less surprised to 

 learn that the ultraviolet rays of the solar spectrum passed through a 

 piece of silver foil, which, we may say, parenthetically, made possible for 

 the first time photography of the invisible. That which is permitted 

 to one ray is prohibited to another. Rontgen's ray, which traverses 

 an oak plank 2 inches in thickness and a plate of aluminum more than 

 a centimeter thick, is stopped by several meters of atmospheric air, 

 the passage of which is but a trifle for the ray of light. 



There is another difference between the Rontgen ray and the lumi- 

 nous ra}' — their conduct in the interior of bodies. Both these rays 

 are absorbed while on their journey; their nature is changed; they 

 are annihilated; their energy is transformed into some other force — 

 heat for instance. This end is common to them. But light has 

 another property which is peculiar to it. In certain bodies having a 

 granular structure, such as roughened glass and the powder of rock 

 crystal, the light is. diffused; the path of the rays is broken by reflec- 

 tions and by numerous refractions. Each particle, then, behaves as a 

 source of light, emitting rays in all directions, and the body is illumi- 

 nated. It would be useless to increase the intensity of the beam of 

 light with the expectation of seeing it transmitted; the illumination 

 would only be increased. 



The Rontgen rays behave very differently. They are only lost 

 through absorption. By increasing the intensity of the rays they will 

 be seen to gain more and more in the power of penetration. They 

 are not diffused. They pursue their path rigidly inflexille, undoubt- 

 edly weakened, but never deflected by any obstacle. A ray of light 

 should not be taken as the type and symbol of ideal rectitude, Init 

 rather the ray of Rontgen. 



There are several varieties of Rontgen rays, as there are of cathode 

 rays. They form an entire scale, and may be distinguished from each 

 other by their degree of penetration. Some are ultrapenetrating. 

 Others are extinguished at a distance of a few millimeters from their 

 origin. This depends upon the generating apparatus, on the current 

 employed, and on other circumstances controlling their production. 



When a Rontgen ray happens to strike a solid body, particularly a 

 metal, it gives rise to rays of the same nature, but having less pene- 

 trating power. They are also much more active from electric and 

 photograph points of view. These secondary rays have been studied 



