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of the mass of the traversing particle to the electron-mass. Williams 

 and Pickup, to whose technique I have already alluded (footnote 7 

 on page 210), observed four tracks of which three were compatible 

 with a rest-mass of about 200, the remaining one requiring a mass- 

 value between 430 and 800. A few more such tracks have appeared 

 in the literature, but instead of describing them I turn for the climax 

 to another and an exacter way in which Fig. 14 furnishes the desired 

 value of mass. 



Fig. 14 — Track of a mesotron slowed down by an obstacle in a Wilson chamber 

 and finally brought to a stop in the gas of the chamber itself. (Neddermeyer and 

 Anderson) 



In Fig. 14, the track beneath the counter comes to a sudden end. 

 One could take a sheet of coordinate-paper, and plot along the hori- 

 zontal axis the curvature of the path as it emerges from the counter, 

 and along the vertical axis the length of the path from that point of 

 emergence onward to its end. This would give a single point of what 

 is known as a " range-vs. -curvature relation" or a " range- vs.-momen- 

 tum" relation. A second point can be found by measuring the 

 thickness of the glass counter-wall twice traversed by the particle, 

 converting it into an equivalent thickness of gas, adding this to the 

 length of the path beneath the counter, and correlating the sum with 

 the curvature of the path at the point where the particle enters the 

 counter. Now, range-vs. -curvature relations are among the best- 

 studied of the features of the charged particles already known — 



