Question of the Homogeneity ofy-Rays. 741 



were in the form of films upon micro- cover glass. Some- 

 times this was placed on the supporting square of micro- 

 cover glass below the preparation. Sometimes the prepara- 

 tion was fastened beneath the support, film downward, so 



Fie-. (>. 



that there was nothing below, but the rays had to penetrate 

 0'25 mm. of glass before entering the electroscope. Thick 

 plates of lead, zinc, and aluminium could be clamped up 

 immediately beneath the preparation at will. The curves for 

 lead, for the hemispherical electroscope (fig. 6) only, are 

 shown in fig. 7 (PI. XII.). The middle curve refers to the 

 bare preparation with nothing below. The upper curve 

 was obtained by placing close above the preparation a disk 

 of lead to reflect back the rays. The lower curve is the 

 difference curve, referring to the reflected radiation only. 

 The middle curve is practically a straight line, if anything 

 slightly concave to the origin. There is thus nothing 

 abnormal about this result due to the use of the hemi- 

 spherical ionization vessel and the cone of rays of angle 180°. 

 Unfortunately no actual comparisons were done for lead, 

 but for zinc and aluminium the results in the hemispherical 

 apparatus were compared with those obtained with the 

 ordinary disposition with cylindrical electroscope described. 

 The curves are shown in figs. 8 & 9 (PI. XII.). In fig. 8, for 

 zinc, the curves A, C, E refer to the hemisphere, the curves 

 B, D, F to the cylinder. As before, the middle curves of each 

 set C and D refer to the bare preparation, the upper A and 

 B to the preparation with a thick plate of lead covering 



