EFFECT OF SCREENS. 13 



Similarly conflicting results were obtained with the screen (fig. 9), 

 the radiation being weaker in view of the greater distance of the bulb 

 from the fog chamber and the more efficient screening. With the 

 lead plates removed, the usual phenomena (oval coronas) appear; but 

 throughout, the contrast is not sharp enough for definite decision. 



Long lead screens placed horizontally within the fog chamber oppo- 

 site the slit in the external screen, as at A in figure n, did not stop 

 the radiation. Elliptic coronas were observed around the trace of 

 the screen as a minor axis, precisely as if the internal screen were 

 absent. Hence either diffusion or secondary radiation must be very 

 active throughout the exposure. 



In experiments 7 to 21 the lead screen, figure 10, was a broad flange 

 on a lead tube, 2.5 cm. in diameter and 4 cm. or 8 cm. long. The 

 X-radiation was usually directed axially, sometimes obliquely, against 

 the walls. With the tube 8 cm. long measurable coronas were not 

 obtained, no matter whether the rays passed axially through the fog 

 chamber or not. In the absence of the lead screen, or when the lead 

 screen was replaced by a thin continuous aluminum screen, the nuclei 

 often filled the chamber on the A and the B sides and the coronas 

 were large. This would again be accepted as evidence favoring the 

 view that the nuclei come out of the walls ; but when the radiation is 

 directed against these walls through the tube there is no appreciable 

 increment. Thus the experiments remain inconclusive. 



The work was now continued by cutting down the tube to 4 cm. in 

 length. The strong coronas obtained were clear and round, with very 

 little fog. At times they seemed to be largest in the middle of the 

 apparatus. Slight oval distortion appeared on the A side near the 

 bulb only. The general absence of distortion when the impact of 

 X-rays is cut off from the top and the bottom again is favorable to 

 an origin of nuclei in those parts. Failure of the experiments 1 8 to 21 

 is attributable to the spark gap of the coil ; but here coronas were 

 often seen throughout the length of the fog chamber, very gradually 

 decreasing in size from A to B. When the eye was moved rapidly in 

 this direction, the coronas were found to lie within a triangle, sym- 

 metrical with respect to the axis of the fog chamber, and the diameter 

 of the coronas vanishes at the apex of the triangle, near the middle 

 of the chamber, as suggested in figure i . 



14. Continued, with change of apparatus. In these experiments a 

 more powerful coil was used, actuated by six storage cells and a 

 Foucault interrupter. The object first aimed at was a contrast of the 

 rays entering the fog chamber axially with the oblique rays which 

 strike the walls. Accordingly, in experiments 22 to 24 the entrance 



