Atomic Nucleus and the Law of Force. ■ 739 



the source, then the number tailing per second on this 

 elementary ring is 



^sin#2.<ty/2. 



The number scattered to unit area at IS placed at right 

 angles to RS is then 



Q • . o 71/-, jL/o * i/o & 2 cosec 4 6 2 



9 V sin cp ; 2 : d<j>l'2 . cos </> 2 . rc< sec <£/2 . - - , r . 



For the whole scattering foil of angular limits (/>t/2, 

 c/> 2 /2 the number of particles falling per second on unit 

 area at 8 is 



QntWCtJ* 

 W K / c°sec 8 <£/2.<ty/2, 



where r' 2 is a mean value. 



Since C o , 1 cos,/: , . , , 



1 cosec 5 xdx= — ~ — „ +4 loo- tan A.r, 

 2 .-m- .r " ° 



the above number is 



Qntb 2 flog tan (£ 2 /i — log tan (^/T + cot c/^/2 cosec <£i/2~| 

 64/2 L —cot <£ 3 /2 cosec (/> 2 /2j ' 



The number of a particles falling per second directly on 

 unit area at S is 



7^75 where i = KS. 



Taking the favourable case of a heavy atom like platinum 

 and ascribing suitable values to the dimensions involved, 

 we find that the scattered number is still only one five- 

 hundredth to one thousandth of the direct number. If the 

 scattered particles w r ere falling on the screen at the rate 

 of 30 per minute, a convenient number for counting, the 

 direct number would be 20,000 per minute. 



The counting of this large number in the direct beam was 

 made possible by a device suggested to me by Sir Ernest 

 Kutherford. If a wheel containing a slit is rotated in the 

 path of a pencil of a rays so that the particles can fall on 

 a ZnS screen only when the slit passes it, the number of 

 scintillations will be reduced in the ratio of the width of the 

 slit to the circumference of the wheel at the position of 

 the slit, and will be independent of the speed of rotation 

 of the wheel. Thus with a slit of 2 mm. in a wheel of 



