In the cloudchamber arrangement previously described 1, 2 the picture (Fig. ) has been obtained whichshows a mesotron track ending in argonof 1.3 atmospheres. The track, curved by a magnetic field of I, 15o oersteds, shows the particle entering a lead plate of I-cm. thickness from which it emerges heavily ionizing. It underoges 2 single scatterings of about 3 Β°each at A and at B, and it ceases to ionize at C. The approximate mass of the particle has been determined in two ways. First, the radius of curvature of the track above the lead plate is x5o 4-5o cm. Combining this with the fact that its range is approximately I cm of lead we find a value of rn = IOO 4-5o electron masses. Below the lead plate the radius of curvature between A and B is 01 = 23 era., and between B and Cit is 02 = I7 cm. The total ranges at the corresponding positions are, respectively, R1 = I2.O cm. and R~= 6.4 cm., from which we find ml = 8o, and m~= 7o. For the computations of m the energy-momentum curves published recently by Rossi and Greisen ~have been used.
The rather large curvatures in the second compartment can be measured with considerable accuracy, but errors are introduced by the multiple scattering which produces additional curvatures superimposed upon those caused by the magnetic field. Williams 4 has deduced the average radius of curvature p, owing to the multiple scattering near the end of a track and finds p. ~R X 1.3(m/Z)l,