electrons from the outer shell of the atoms and 

 electrons separated from atoms or "free elec - 

 trons. " The process of formation of X rays 

 takes place in a vacuum tube filled with rarefied 

 gas where by application of high voltage across 

 the tube, free electrons are split off from indi- 

 vidual atoms and directed at a very high speed 

 toward the anode, thus forming a beam of "cath- 

 ode rays. " The electrons are propelled in the 

 tube at a speed approaching that of light. In 

 order to impart such speed to electrons it is 

 necessary to expend a large amount of energy, 

 which is derived from a special source of cur- 

 rent. If a metal plate is placed in the path of the 

 electrons, they will strike it with tremendous 

 force; their speed diminishes abruptly, and at 

 that instant X rays are formed. This plate has 

 to be made from a metal with a very high melt - 

 ingpoint since electronic bombardment generates 

 a large amount of heat. The plate is called the 

 anode of the tube and the tube itself an X-ray 

 tube. The most prevalent type of electronic tube 

 is the so-called Coolidge tube which was made 

 first in the United States in 1913. The X-ray 

 tube is essentially a container from which the 

 air has been pumped. Inside the container are 

 two electrodes; one is connected to the anode, 

 the other to the cathode, which constitutes the 

 source of electrons (Fig. 1.) X rays are short 



wave electromagnetic radiations ranging in 

 wave length from 0.02 to 100 angstroms, a 

 unit of length equivalent to 1/10, 000 jli or 

 1/lOm/j.. The energy radiated by the tube is 

 not all of the same kind, it consists of both 

 short and long wave radiation. The qualitative 

 composition of the radiation depends on the 

 voltage: the greater the voltage, the greater 

 the quantity of short wave rays produced. 



X rays are invisible and can be detected only 

 by their action. Three basic means are used to 

 detect rays: 1) fluorescence, 2) photography, and 

 3) ionization. The first method is based upon 

 the ability of X rays to cause visible lumines- 

 cence in certain substances. If a screen is made 

 from such substances, it will light up every time 

 that X rays impinge upon it (fluoroscopic exami- 

 nation is based upon this principle). The photo- 

 graphic action of X rays is similar to fluores- 

 cence but more pronounced. It is possible, for 

 example, to obtain photographic prints by means 

 of X rays. These prints can be retained as doc- 

 umentary evidence for an indefinite period. 

 Kostov (1943) has had great success in photo- 

 graphing ears of wheat by means of X rays, a me- 

 thod that enables him to obtain not only the ex- 

 ternal morphological appearance of the ears, but 

 even the extent of development of the kernels inside 



;#&*» 



FIGURE 1. X-ray apparatus in action. 



