392 PROTOPLASM 



different wave length produce different effects on human beings, 

 especially when the rays He beyond the two ends of the visible 

 spectrum. The actinic qualities of the ultraviolet are much 

 greater, as measured by their influence on human skin, than are 

 those of the infrared. It is the ultraviolet which tans and blis- 

 ters the skin. Ultraviolet light extends from the violet end of 

 the visible spectrum at 3,900 A. U. to below 1,000 A. U. Beyond, 

 but overlapping the ultraviolet slightly, lie the X rays. 



Just as the wave lengths of the infrared rays ultimately 

 become, upon a gradual increase in length, those of radio rays, so 

 do those of the ultraviolet, on a decrease in length, become X rays. 

 When shortest — between 1 and 0.1 A. U. in length — X rays are 



The Crookes or X-ray tube. 



most penetrating and are then termed "hard." At this length, 

 they become gamma rays, the usual source of which is the dis- 

 integration of atomic nuclei. X rays were discovered by Roent- 

 gen, by whose name they are also known. X rays are produced 

 in a cathode tube and come into existence when cathode rays 

 strike the anticathode, the anode itself, or any other surface, as 

 the result of high electrical tension between the cathode and anode 

 poles of the tube. The original laboratory source of X rays was 

 the Crookes tube (Fig. 167), which has now developed into the 

 Coolidge tube, a more compact and much more efficient type, 

 wherein other forms of radiation have been eliminated to a great 

 extent. 



As in the case of ultraviolet light, X rays do not make an 

 impression upon our eyes, but if a fluorescent screen of platinum 

 barium cyanide is placed in their path, a brilliant glow results. 

 The same effect is obtained when invisible ultraviolet strikes a 

 fluorescent screen. This similarity in the behavior of ultraviolet 

 and X rays and the fact that both affect the photographic plate 

 indicate that X rays are identical with or of the nature of light 

 rays, yet when first studied they could not be diffracted or scat- 



