CONTEMPORARY ADVANCES IN PHYSICS, XXVIII 581 



be brought about by its encounters with atoms and the electrons which 

 atoms contain; and there would be statistical variations between the 

 numbers of atoms and electrons which different particles would 

 encounter after plunging into such a sheet. The probable effect of 

 these variations can be computed; and it was shown at an early date 

 that for at least some of the substances emitting alpha-rays — a~ 

 emitters — the diversity in ranges of the particles is no broader than 

 should be expected. The curve of distribution-in-range of an a-ray 

 beam often consists of a single peak or hump, and the shape and 

 breadth of the hump are consistent with the assumption that it is 

 due entirely to the "straggling" (the name applied to the statistical 

 variations aforesaid) of particles all possessed initially of a single 

 speed. 



The vanishing of this beautiful but too-simple theorem from physics 

 is due to experiments of three types. First, it was found that when 

 all of the well-known a-particles of about 8.6 cm. range from ThC 

 were completely intercepted by a stratum of matter of rather more 

 than 8.6-cm. A.E. (air-equivalent ^2), and the detecting apparatus 

 was adjusted to a sensitiveness much greater than would have been 

 tolerable for the main beam, a very few particles — a few millionths 

 of the number in the 8.6-cm. flock — were still coming through. Some 

 of these had ranges as great as 11.5 cm., immensely greater than 

 could be ascribed to straggling. These are the "long-range" alpha- 

 particles, other examples of which have been discovered with RaC 

 and (very lately) with AcC. Next the colossal new magnet at 

 Bellevue near Paris was employed by Rosenblum for deflecting a- 

 particles and observing their velocity-spectrum, and the unprecedented 

 dispersion and resolving-power (to employ optical terms) of this 

 superb apparatus disclosed that for several a-emitters (the list now 

 comprises eight) the spectrum consists of two or several lines instead 

 of only one. The "groups" of alpha-particles to which these lines 

 bear witness lie closer to one another in energy than the aforesaid 

 long-range particles lie to the medium-range ones, wherefore they are 

 often said to constitute the "fine-structure" of the alpha-rays; but it 

 is probable that a more significant basis for distinction lies in the fact 

 that the long-range corpuscles are relatively scanty, while the various 

 lines of a fine-structure system are not so greatly unequal in intensity. 



'2 1 recall that while the range of an alpha-particle of given speed depends on the 

 density and nature of the substance which it is traversing, the student is usually 

 dispensed from taking account of this by the fact that the investigators nearly always 

 state, not the actual thickness of the actual matter which they used, but the thickness 

 of air at a standard pressure and temperature (usually 760 mm. Hg and 15° C.) 

 which would have the same effect. 



