﻿46 Mr. Gr. H. Henderson on the 



straggling are three or four times greater than those predicted 

 by theory. Furthermore the calculated straggling increases 

 steadily with increase of range, while that observed is 

 constant within the limit of error. It should be pointed out 

 that the projections given in the Table are measured as the 

 small differences between two larger quantities, and hence 

 are more difficult to determine with accuracy. The straggling 

 deduced from the writer's experiments has a probable error 

 of about 2 per cent., and it will be seen that the values for 

 the three types of rays agree within this limit. 



It was shown by the writer (loc. cit.) that the effect of 

 straggling due to electronic encounters would be a tailing 

 off' of the ionization curve at the extreme end of the range. 

 Making some simple assumptions it w r as shown that the 

 calculated form of the end of the ionization curve agreed 

 satisfactorily with the form of the curve observed between 

 A and B (fig. 1). Thus the effect of the calculated straggling- 

 was amply accounted for by AB, leaving the much greater 

 straggling evidenced by the straight line portion BC quite 

 unexplained. The curves obtained by counting experiments 

 also lead to precisely the same conclusion. In view of the 

 failure of theory to account for this large excess straggling- 

 it is interesting to see what further information regarding it 

 can be derived from experiment. 



It is remarkable that the straggling (as measured by the 

 projections of the ionization and also of the counting curves) 

 should be constant for a rays differing so widely in range as 

 those given in Table I. This can only mean that the excess 

 straggling takes place only in the last tw r o or three centi- 

 metres of the range. From experiments with gold foils 

 which will be discussed later, it appears probable that the 

 straggling is confined to the last few millimetres of the 

 range. 



Referring once more to fig. 1, it could not be expected 

 that the straggling deduced from the ionization and the 

 counting data would agree, for the following- reasons : — The 

 ionization curve is regarded as being built up of simple 

 curves of different ranges grouped about a common mean. 

 The form of the simple curve is not accurately known ; the 

 rule that the ionization is inversely proportional to the cube 

 root of the remaining range can only be an approximation to 

 a much more complicated law. As the shape of the simple 

 curve cannot be taken into account, the projection of the 

 ionization curve which is actually utilized can only give a 





