November 25, 1921] 



SCIENCE 



523 



swung about by tbe strong field there encoun- 

 tered. For an electron to tbus enter and 

 emerge from an atom without appreciable loss 

 of energy would seem to require that it do so 

 without having a near encounter with any of 

 the electrons of the atom structure. On the 

 other hand the structural electrons may be so 

 anchored in position that no energy is trans- 

 ferred to them in any except very close encoun- 

 ters. The fraction of the primary electrons 

 scattered from a nickel target without appre- 

 ciable loss of energy is small, not more than 

 one in a thousand being turned back with a 

 loss not to exceed one per cent, of its initial 

 energy. 



The distribution of these high-speed scat- 

 tered electrons in the region in front of the 

 target is particularly interesting. Our observa- 

 tions suggest that it is entirely symmetrical 

 with respect to the incident beam and inde- 

 pendent of the inclination of the target to the 

 incident beam except as this affects the region 

 into which the scattered electrons are free to 

 emerge. With a target inclined at an angle of 

 45 degrees to the incident beam the intensity 

 of scattering as a function of angle has been 

 studied in the plane including the incident 



beam and the normal to the target. The range 

 of 135 degrees on one side of the incident beam 

 has been explored with the exception of 25 de- 

 grees adjacent to the beam. The Faraday box 

 collector used for picking up the scattered elec- 

 trons can not be brought nearer the primary 

 beam in our present apparatus. The principal 

 features of the angular distribution are two 

 maxima of emission, one back along the path 

 of the bombarding electrons (^ = 0) and an- 

 other lateral to the primary beam whose posi- 

 tion depends upon the bombarding voltage. 

 The relative importance of these two maxima 

 also depends upon the speed of the primaries. 

 Fig. 1 shows such a distribution curve for 

 a bombarding potential of 150 volts. The 

 intensity is measured as the ratio of the 

 current entering the Faraday box collector 

 to the total current reaching the target. The 

 opening in the Faraday box subtends about 

 .03 of unit solid angle to the spot under bom- 

 bardment. The retarding potential between 

 box and target for the curve. Fig. 1, is • 135 

 volts, so that only electrons that have lost 

 not more than 10 per cent, of their initial 

 energy are caught. The effect of bringing the 

 retarding voltage nearer the bombarding volt- 



