570 



Sir E. Rutherford on Collision of 



short time the skilled assistance of Professor E. Marsden 

 before his retnrn to New Zealand. Systematic observations 

 were undertaken to record electromagnetically the time of 

 appearance of each scintillation on a chronograph tape while 

 the number of doubles was separate!}' recorded. Mr. Marsden 

 and Mr. Kay counted alternately for a minute interval, and 

 the counts of each observer were separately analysed by 

 the former at leisure. On the probability theory, the 

 number of intervals between tj and t 2 seconds is given by 

 ^ e -^ti-t2) where 1//jl is the average interval between each 

 scintillation and N the total number of intervals. Marsden 

 and Barratt* had previously verified the correctness of this 

 theory, which shows that short intervals are more probable 

 than long ones. If the average number of scintillations is 

 30 per minute, /i=l/2, and iE the eye fails to distinguish an 

 interval less than 1/10 of a second, the average number of 

 doubles to be expected is 1*5 per minute. In practice, under 

 favourable conditions, the eye is just able to detect 1/10 

 second intervals for bright a ray scintillations. 



Comparisons were made to test the probability distribution 

 of a. particles from polonium, whose range was adjusted to 

 give a scintillation of about the same average brightness as 

 the H atom. 



The results of a typical series of counts both for a rays 

 and H atoms are included in the following table. The theo- 

 retical and observed number of intervals <1/10, < 1/2, and 

 < 1 second are given in the table below : — 



Iverage 



Total 



Number 



of 

 doubles. 



Calculated 



Observed 



Calculated 



Observed 



Calculated 



Observer. 



numDer 



number 



number of 



number of 



number of 



number of 



number of 



scintil- 



of scintil- 



intervals. 



intervals. 



intervals. 



intervals. 



intervals. 



lauons 

 per min. 



lations. 



< 1/10 sec. 



< 1/2 sec. 



< 1/2 sec. 



<1 sec. 



<1 sec. 



a. partic 



Ies from 



poloniu 



m. 













M .. 



28-0 



2S0 



13 



129 



53 



60 



106 



105 



K ... 



254 



229 



10 



9-H 



45 



45 



83 



79 



Hydrog 



en atom 



s. 















M ... 



24-3 



213 



15 



9-6 



50-5 



46 



84 



81 



K ... 



227 



250 



25 



9 2 



58-5 



45 



92 



79 



M ... 



31-0 



216 



24 



11 



60-5 



50 



95 



88 



K ... 



29-6 



148 



18 



7 



33 



35 



59 



58 



1 



* Marsden and Barratt, Proc. Phys. Soc. xxiii. p. 367 (1911) ; xxiv 

 p. 50 (1913). 



