CONTEMPORARY ADVANCES IN PHYSICS 135 



annoying, but it was quite an achievement so to adjust the conditions 

 as to get so few as three.) Many splendid examples have lately been 

 published by Dee and Walton of the Cavendish, and Fig. 14 is out- 

 standing among them because the bombarding stream was a mixture 

 of protons and deutons, and the picture shows two pairs of fragments, 

 one apparently due to each of the processes which I have been de- 

 scribing. Those of the pair marked &1&2 have the range of 8.4 cm. 

 agreeing with equation (1), while those marked aia2 go definitely 

 farther and even escape from the chamber, which makes it impossible 

 to measure their ranges. Dee and Walton therefore made the walls 

 of the target-capsule thicker, so that more of the energy of the frag- 



Fig. 13 — Tracks of paired fragments, He nuclei resulting from impact of a proton on a 

 Li' nucleus. (Kirchner; Bdyrische Akademie) 



ments should be consumed in them; the pairs which were obtained 

 with bombarding deutons now ended in the chamber and in the field 

 of view, and their ranges agreed with the 13.2 cm. obtained from the 

 curve of Fig. 9. At least two more of these pairs appear in Fig. 15. 

 Verification of a theory could scarcely go further or be more vivid ! 

 Yet there is the additional point, that Kirchner found the angle 

 between the paired paths in his pictures to differ from 180° by just 

 about the amount required by the momentum of the proton. 



However not every fragment observed when lithium is bombarded, 

 either by protons or by deutons, results from these superbly simple 

 interactions. Notice in Fig. 8 the two very much rounded steps, 

 suggesting groups of short ranges (1.15 cm. and 0.65 cm.); these are 

 confirmed by the maxima in the curve of Fig. 10 which has already 



