NUCLEAR PHYSICS — DUBRIDGE 239 



the counter itself; and they also respond to cosmic rays. This means 

 there will be a background of from 5 to 25 counts per minute which 

 must be carefully determined and subtracted from the total observed 

 with the source in place. It is this background which limits the 

 ultimate sensitivity of the device, for the source being tested should 

 produce a number of counts per minute at least as large as the back- 

 ground. Statistical fluctuations become important for low counting 

 rates but can be reduced by extending the time of counting. A total 

 of 10,000 counts must be made if the precision is to be as high as 

 1 percent, but only 100 counts will give 10 percent precision. 



It is thus evident that simple readings of a clock and a registering 

 device plus a little arithmetic is all that is involved in following 

 atoms from place to place through even a complex biological system. 

 It has been the experience that biologists who use this technique 

 for the first time become wildly excited over the relatively simple way 

 in which studies of most fundamental importance can be carried out. 

 I shall expect this experience to be often repeated during coming 

 years. 



