298 HELL SYSTEM I ECIISICAL JOIRX.IL 



"max- The \alue' given for il by Gerlacli, after a critical stii(i\- of 

 all the determinations, is 



/z=().53.10-=' (4) 



The highest frefiiiency of radiation which electrons moving with 

 the energy E are able to excite, when they are brought to rest b\- 

 colliding with a metal target, is therefore equal to E divided by a 

 constant indei)endent of the kind of metal. So far as this high lim- 

 iting frequency is concerned, it is perfeclK' legitimate to express 

 equation (3) in these words. 



Excitation of radiation by electrons stopped in their flight by collision 

 with a metal occurs as if the energy in the radiation were concentrated in 

 units of amount hv, and one such unit were created out of the total energy 

 which each electron surrenders when it is stopped. 



As for the radiation of frequencies inferior to the high limiting 

 frequency, it is very easily explained by asserting that most of the 

 electrons come to rest not in one operation, but in several successi\e 

 ones, dividing their energy up among several units of frequencies 

 inferior to ^nuix or £ /;; or possibly they lose energy in \arious sorts 

 of impacts or various other ways before making the first impact of 

 the sort which transforms their energy into energy of X-rays. Xoihins.; 

 about il contradicts the italicized rule. Still it is not likely tli.it ,iii\- 

 one would have formulated e(|ualion (3) in such language, if the \alue 

 of the constant /; which appears in it were not identical with the value 

 which we have alread\' once encountered in anaKzing the jihoto- 

 electrir effect, and with the \'alue at which Planck earlier arri\-ed. 



I think it is too early in this discourse to fuse these italicized Rules 

 for the release of electrons by radiation and the excitation of radiation 

 by electrons into a single Rule; Inn by cnniomplaiing the two Rules 

 side by side one arrives without niucli labor ,u an inference which 

 could be tested e\en though we had no wa\' of measuring the fre- 

 quency of a radiation, and in fact was \-erified before any such wa>' 

 existed. For if electrons of energ\- E can excite radiation of fre(iiienc\' 

 E/h, ami radiation of frequency E, h striking a piece of metal can 

 elicit electrons of energy // (/i, h)—P; then, if a target is bombarded 

 with electrons, and another metal target is exposed to the radiation 

 which emanates from the first one, the fastest of the electrons which 

 escape from the second target will mo\e with the same velocity and 



■ tii-rlaih ri-(;ar<ls this as the most arcuratc of all ihf methods for (Iclcrmhiiiig //, 

 un upliilon in which probably \vA all would concur. Il has been nialntaine(l that 

 the hiKh-fre(|uency limit, like the wavelength of niaxinuini intensity in the X-ra\- 

 siH-ctrum, deiH-nds on the inclination of the X-ray beam to the exciting electron- 

 stream. I do not know whether the exi>erimenls adduced in support of this claim 

 h-n.- 1 n -mI.- K ,,.„t, ,..■.! 



