4 Art. 10. — S. Kinoshita and H. Ikenti : 



does not appear distinct from the otliers along the whole length, 

 but, in the vicinit}^ of the centre, comes very close to or overlaps 

 the neighbouring ones, even when the radiant nucleus is 

 practically a point. This effect also gives rise to a dark area at 

 the centre, which has no definite shape. The dimensions of the 

 dark area vary of course with the number of the trails. 



3. Two different types of radial a ray tracks. As microscopic 

 examinations at once show, the trails of grains may, in general, be 

 divided into two difïerent types. 



In the first type, the trails emerge directly at the centre or, in 

 case wdien the dark nucleus is present, at the rim of it. Conse- 

 quently, when a set of homogeneous a rays have been utilized, all 

 the tracks terminate very nearly at the circumference of a circle 

 drawn around the centre, and present themselves as a halo (figs. 

 2-8), resembling the pleochroic halo seen in minerals such as 

 biotite, and investigated in detail by Joly^\ 



Without question, the radial tracks do not all lie on planes 

 parallel to the surface of the emulsion film, and as can Ije traced 

 by focussing the microscope, some of them are running oblique to 

 the surface. Since the film on ordinary photographic plates has a 

 thickness equivalent to only about two centimetres of air in stop- 

 ping a rays, the photographic halo obtained on these plates can 

 never be that of a complete sphere. 



The trails belonging to the second type spread out around the 

 centre over a wide region of no definite boundary, as can be seen 

 in fig. 9, which is reproduced in the same magnification as the 

 haloes in figs. 2-8. In this case, most trails are found to have 

 their seat within the uppermost layer of the film, showing that 

 they are the tracks of the a particles projected tangentially to the 

 surface of the film from the part of the source just above the sur- 

 face. At first sight, some of the trails seem to be much longer 

 than those constituting the haloes. Closer inspection, however, 

 shows that this is only apparent and that each of them consists of 

 two or more elementary trails following one another in succession. 



1) J. Joly, Phil. Mag. 13 (1907), p. 381 ; 19 (1910), p. 327; J. Joly and A. L. Fletcher, Phil, 

 Mag. 19 (1910), p. 630. 



