January 24, 1913] 



SCIENCE 



129 



though these two velocities sliould be defi- 

 nitely proved to be the same, Bragg 's argu- 

 ment for some sort of a corpuscular theory 

 of X-rays would still stand. 



But, aside from the minor difficulty of 

 accounting for the so-called polarization of 

 X-rays, that is, the dissymmetry of their 

 emission about the point at which they 

 originate, Bragg 's theory encounters the 

 supreme difHculty of accounting for the 

 rapidly growing evidence of a complete 

 parallelism between optical and X-ray 

 effects. Thus : 



(1) Ultra-violet light, like X-rays, ejects 

 electrons with speeds which have been re- 

 peatedly shown to be completely indepen- 

 dent of the intensity of the source. I have 

 myself raised a doubt about this conclusion, 

 but have recently shown that the doubt is 

 unjustified,-' and that the conclusion holds 

 even when the intensity varies in the ratio 

 1,000 to 1. 



(2) In the normal photo-electric effect, 

 which has none of the earmarks of a reson- 

 ance phenomenon, all observers now agree 

 that the speeds of the ejected electrons in- 

 crease regularly with the frequency of the 

 light,-^ just as the speeds of electrons 

 ejected by non-homogeneous X-rays in- 

 crease with the hardness of the rays. 

 Apparently, too, the law of increase is the 

 same in each case. 



(3) There is a selective photo-electric 

 effect characterized by the emission at a 

 particular frequency of the exciting rays of 

 an abnormal number of electrons. This 

 emission can not be excited until the fre- 

 quency of the incident light reaches a 

 definite value which is characteristic of the 

 illuminated substance. This selective ef- 

 fect bears all the earmarks of an absorp- 



-' Physical Beview, January or February, 1913. 



^ E. Ladenburg and K. Markan, Phys. Zeit., 9, 



p. 821 ; Hughes, Phil. Trans., CCXII., p. 205, 1912. 



tion band.-" Precisely similarly there is a 

 selective X-ray effect characterized by the 

 emission at a given hardness of an abnormal 

 number of electrons, and also by the ex- 

 citation of a new type of X-ray radiation, 

 which differs from the ordinary or scattered 

 X-ray in being homogeneous, symmetrical 

 about the origin, and having a penetrating 

 power which is characteristic of the emit- 

 ting substance instead of the quality of the 

 exciting X-ray. This so-called homogene- 

 ous or characteristic X-radiation can not 

 in general be excited until the hardness 

 of the exciting ray exceeds a definite 

 value. This critical value is nearly pro- 

 portional to the atomic weight of the ex- 

 cited substance. The exciting rays experi- 

 ence absorption at the hardness at which 

 the new increase in /8-ray emission occurs. 

 In other words, this selective X-ray effect, 

 like the selective photo-electric effect, bears 

 all the earmarks of an absorption band.^° 



(4) Light rays. X-rays and y-rays all 

 behave exactly alike in throwing more elec- 

 trons forward in the direction in which the 

 rays are moving than backward in the 

 direction from which they came.^^ 



(5) Finally, Laue, Friedrich and Knip- 

 ping,^- by using as a diffraction grating the 

 regular arrangement of the molecules 

 themselves in a crystalline substance, have 

 recently obtained beautifully sharp photo- 

 graphic patterns which resemble very 

 closely diffraction patterns in light. The 

 wave-lengths of the X-rays computed from 

 assumed intermoleeular distances is about 



'" PoM and Pringsheim, Verh. d. D. Phys. Ges., 

 1911 and 1912. 



=°BarkIa and Sadler, Phil. Mag., May, 1909; 

 Sadler, Phil. Mag., March, 1910; Whiddington, 

 Proc. Boy. Soc, 1911 and 1912. 



"■ Bragg, ' ' Studies on Radioactivity ' ' ; Kleeman, 

 Proc. Boy. Soc, 84, p. 93, 1910; Stuhlmann, Phil. 

 Mag., 20, p. 331, 1910; and Robinson, Phys. Zeit., 

 13, p. 276, 1912. 



^Mimch. Per., pp. 303-322, 1912. 



