14 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



of radiation into the axial part of the end of the fog chamber is cut 

 off by a lead disk, D, figure 12, about 2.5 cm. in diameter. In com- 

 parison with experiment 27, where the screen was removed, the coronal 

 effect for the disk is somewhat weaker, but throughout of the same 

 nature, with campanulate or spindle-shaped coronas filling more than 

 half of the length. Much rain and fog were present, and crimson- 

 colored streamers stretched horizontally and symmetrically , bow-shaped 

 (convexity outward), from end to end. The first coronas (s^) are not 

 measurable, but, after the first fog particles have fallen out, the 

 second (s 2 ) are quite so, being round and clear. 



FIGS. 11-12. Fog chambers with screens and X-ray bulb. 



The disk was now removed and a lead screen, S, figure 1 1 , with a hole 

 about 2.5 cm. in diameter, placed over the end of the fog chamber, with 

 the X-ray bulb placed as before. To make the wood more transparent 

 a waxed cork was inserted, giving free entrance to the axial rays. All 

 screens were earthed as usual. Experiments 25 and 26 show the 

 results. The contrast with the preceding is marked. The coronas 

 are round, and in the first exhaustion show apertures (-O decidedly 

 smaller than the coronas obtained in the second exhaustions of the 

 preceding cases. True, the amount of radiation entering the fog 

 chamber is much larger for the case of the disk than for the case of the 

 perforated screen; but it nevertheless follows that the axial rays, even 

 if entering under favorable conditions, can not be specially efficient. 

 Rays which have penetrated the fog chamber obliquely and impinge 

 on the top and bottom are responsible for nearly the whole of the 

 dense fog usually observed. 



In further experiments, work with the flanged lead tube (2.5 cm. in 

 diameter, 4 cm. long, figure 10, center of bulb 5 to 6 cm. from the end 

 of the fog chamber and about 8 cm. from the inner face) was resumed 

 and successively placed in positions, a (axial), / (radiation grazing the 

 top surface), b (radiation grazing the surface of water below, as seen 

 in figure 1 1), the screen being moved with the bulb. The experiments 



