SO.Ml'. COS'TF.Ml'OR.tRy .HH.IXCr.S /.V I'lnsiCS 11 165 



the luirk-us, ;iiul st>n\e of them ejrct floctroiis from tlie A.'-li'\rl of tlic 

 atom, thus producing the neci-ssai^' condition for the ^n-radiation 

 and all the others to be emittetl. These electrons from the A'-level 

 Wf)uld escape with too little energy to he registered in the apparatus. 

 The ciueslion of the ultimate origin of these fastest electrons is, how- 

 ever, still under debate by the leading authorities on the subject. 



1-ig. 6 



Imagine now that a beam of X-ra\s iiukiiliii!; all frecjuencies is 

 directe<l ag.iinst a thin sheet of metal atoms, and that the transmitted 

 beam is dispersed into a sf>ectrum projected against a photographic 

 plate in the usual manner. Rays of frequency v can extract electrons 

 from a particular level when ht> exceeds the value of W for that level, 

 but not otherwise. Advancing along the spectrum in the direction 

 of increiising frequencies, we should expect to find a sudden sharp 

 weakening of the transmitted rays wherever the frequency becomes 

 equal to one of the values W/k which characterize the various levels. 

 -Some of I,, de Broglie's classical photographs are shown in Fig. 7 

 (borrowetl from Millikan's book, "The Electron"). The second 

 picture from the top represents two spectra of an X-ray beam trans- 

 mitted through molybdenum, one spectrum stretching away to the 

 right from the central dark band, the other to the left. The fre- 

 quency decreases as the distance from the dark band increases. Com- 

 ing inwards toward the band, we see that the plate very suddenly 

 becomes whiter at a certain critical frequency; this is the frequency 

 at which hv becomes equal to the IF of the /l -level. Similar spectra 

 of beams transmitted through cadmium, antimony, barium and mercur>' 

 are presented below the molybdenum spectrum; the corresponding 

 absorption-etlge is discerned in each, its frequency rising with the 

 atomic number of the element.* This is by far the most delicate and 

 accurate method of determining the various extraction-energies, 



* The topmost picture shows the spectrum of the team before it encounters the 

 absorbing layer; the various strong Hnes in it, and the absorption-edges impressed 

 upon it by the silver and bromine atoms in the photographic (ilm, recur more or less 

 clearly in the absorption-spectra, but have nothing to do with the atoms in the 

 absorbing layer. 



