﻿50 



Mr. G. H. Henderson on the 



placed behind these four again only slightly increases the 

 straggling. 



The same result was also demonstrated in a slightly 

 different manner. A foil made up of the four gold leaves 

 already used was placed at different distances from the 

 ionization chamber and the straggling determined from the 

 ionization curves as before. The results are shown in 

 Table III. The distances given in the first column are not 

 the actual distances from the foil to the centre of the 

 ionization chamber, but are reduced to correspond with an 

 ionization vessel containing air at atmospheric pressure. 



Table III. 



Distance from 

 Ionization Chamber. 



Straggling. 



•7 mm. 

 4-7 „ 

 12-9 „ 



3'65 mm. 

 3-26 „ 

 2'95 „ 



Here again it can be seen that the straggling increases 

 rapidly near the end of the range. 



It may also be noted that the straggling (2*95 mm.) when 

 the foil was 12*9 mm. from the ionization chamber is less 

 than that (3 01 mm.) obtained when the foil was placed 

 directly over the source. The difference is less than the 

 possible error, but such a change is to be expected. For 

 the straggling due to unevenness of the foil should be less 

 for low velocities of the a particles on account of the decrease 

 in air equivalent of the foil. For the same reason the differ- 

 ences given in the fourth column of Table II. are probably 

 slightly too small. 



From the straggling observed with different types of 

 ol particles it was pointed out in § 2 that the straggling must 

 occur within the last two or three centimetres of the range ; 

 from the results with gold foils it seems that the straggling 

 must be confined largely to the last few millimetres of the 

 range. 



The increase of straggling due to gold foils placed near 

 the end of the range, though clearly marked, is small. 

 This is quite in accordance with the view expressed in § 3 

 that the straggling does not increase rapidly with increase 

 o£ stopping power of the substance causing the straggling. 

 It must be remembered that the increase - observed is the 



