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TEXTBOOK OF ZOOLOGY 



oil in the treatment of rickets. It appears that in the body the 

 sunlight acts on the sterol-bearing fats of the skin to produce the 

 necessary vitamin D. 



Roentgen Radiation 



In 1895, W. C. Roentgen, a professor of physics in the University 

 of Wiirzburg (Bavaria), discovered that the cathode ray tube with 

 which he was working threw some sort of invisible radiation on a 



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Fig. 370. — Diagram of a roentgen ray machine arranged to irradiate some 

 biological material. The line current is comparatively low and has to be stepped 

 up by the transformer. The input is usually alternating current and must be 

 changed by the rectifier to direct current. The roentgen ray tube is of glass, is 

 evacuated, and contains a filament at the cathode end and a target at the anode 

 end. The filament is heated to incandescence by means of a low voltage current 

 and gives off electrons. A high negative potential is applied to the filament and 

 this drives the electrons across the tube where they are attracted to the positively 

 charged target. The energy of the electrons is given up to the target (usually 

 tungsten) as they are stopped and part of this energy is emitted as roentgen 

 radiation. 



barium platinum cyanide screen, causing it to glow. He found that 

 when his hand was placed between the tube and the fluorescent 

 screen that bones would absorb the radiation while the softer tissues 



