CONTEMPORARY ADVANCES IN PHYSICS 303 



Several other such photographs were taken by Blackett and Occhialini 

 in Cambridge (England) and by Anderson in Pasadena. 



Events of another type are observed, when the mixture of neutrons 

 and photons emitted from beryllium bombarded by alpha-particles is 

 allowed to fall upon a metal plate: the tracks of many ionizing cor- 

 puscles are noticed springing from the plate, and when there is a 

 magnetic field applied, some are seen to be curved one way and some 

 the other. Yet another is exemplified in Fig. 2. This is a historic 

 photograph, the one from which the positive electron was first inferred 

 (by C. D. Anderson) ; it is rarely that one can fix with such precision 

 the moment of a major discovery, and perpetuate the very observation 

 out of which it was made. Here obviously is the path of a single 

 particle coming from below, which has cloven entirely through the 

 lead plate of 6 mm. thickness, and has emerged from the upper side 

 with diminished speed revealed by the augmented curvature of the 

 trail. It is this change of curvature which fixes the sense of the 

 traversal of the path, and the sign of the curvature thereupon fixes 

 the sign of the particle's charge as positive. 



But granted that many of the ionizing corpuscles which interlace 

 the air are positively charged: are they not simply alpha-particles or 

 protons, or of some other well-known type of positive ion.'* Here 

 enters the second item of the evidence. Assuming (e.g.) the agent of 

 the trail of Fig. 2 to be a proton, one may calculate the speed which 

 it would necessarily have, in order to suffer a curvature-of-path equal 

 in magnitude to that which is observed. One may then evaluate, 

 from prior knowledge, the number of ion-pairs per unit length of trail 

 which it would produce ; and this turns out to be many times as great 

 as that which is observed. A proton would produce a trail much 

 denser, and also much shorter, than the actual one; its energy would be 

 used up in a progress of 5 mm. away from the plate, whereas the visible 

 course of this corpuscle extends for more than 5 cm. and shows no 

 sign (in thickening or in increase of curvature) of being near its end. 



The particle of Fig. 2 was therefore not a proton, nor, a fortiori, an 

 alpha-particle or more massive ion; and the only way to reconcile the 

 observed curvature wdth the observed density of path seems to be, 

 to assume a particle about the same as the electron both in mass and in 

 magnitude of charge, though not in sign of charge. This is not the 

 same as saying that either the charge or the mass is accurately de- 

 termined. Apparently it is certain that the charge must be less than 

 -\- 2e, which makes it equal either to le or to some non-integer multiple 

 of e, and the latter alternative is too painful to be borne. As for the 

 mass, it must be many times smaller than that of the proton (if the 



