Jib HLI.L SYSTEM TECIIMC.U. JOIKX.U. 



location is the measure of the wave-length. An image can be made 

 on the same plate at the point where the beam would have struck it, 

 if it had retained the frequency of the primary beam. The two 

 images then stand sharpK- and widely apart. Indeed it is not neces- 

 sary to make a special image to mark the place on the plate where 

 a scattered beam of unmodified wave-length would fall, for there 



Fig. 6 — Diagram sliowing the energy-relations ensuing upon an impact between a 

 riuanlum and a free electron. {.After Uebye.) See footnote 13 



nearly always is such a beam aiui such an image. .\ plausible ex- 

 planation is easy to find; one has onh' to assume that the ciuania 

 composing this beam have rebounded from electrons so rigidK' bound 

 into atoms that they did not budge when the impinging quanta struck 

 tiuiu. ,uid liusi' were reflected as fnnii an inimoxabli' wall." 



" The iliagrani in I'ig. 6 is designed to illustrate the relations between the energy 

 of the primary' quantum (radius of the dotted semicircle), the encrg>' of the re- 

 bounding (juantum (radius of the upper continuous curve), and the energy of the 

 recoiling electron (radius of the lower continuous curve). Thus the two arrows 

 marked with a .S are proportional respect i\el\' to the energies of the secondary 

 quantum and of the recoiling electron, when the encounter has taken place in such 

 a fashion that the angle 9 is equal to the angle lietween the arrow 10 and the upper 

 arrow .S. In the same case, the angle between arrow 10 and_ lower arrow 5 is equal 

 to tp of the ecpiations (9). 



" .\s a matter of fad we have no Independent mc.ins of knowing that the recoiling 

 electrons are initially free, or that t-lie scattered be.im with the modified frequency 

 originates from collisions of primary quant. i with initi.dly free electrons; we know 

 <>nly thai the frequency of the scattered (pianta is such as would be expected if 

 little or no energy is s|H-nt in freeing the electrons, and little or no momentum is 

 transferred otherwise than to the electrons — which, of course, is not quite the same 



