Alpha Particles from Liglit Atoms. 539 



Counts were rarely continued after 80 minutes, as the small 

 numbers of scintillations caused too much uncertainty. At 

 the end of 26 counts or less the writer's eyes had become too 

 fatigued to continue. The writer desires to emphasize to 

 future workers the necessity of: resting the eyes two or three 

 days between counts. A problem of this nature should be 

 undertaken by two observers, counting alternately, the one 

 doing the technical manipulations while the other is in the 

 dark. Owing to the shortage of time the writer, who counted 

 alone, was forced to make as many as six counts a week. At 

 the end of two months the writer's eyes had become com- 

 pletely insensitive and counts very erratic. This condition 

 was also accompanied with all the other symptoms accom- 

 panying severe eye-strain, and should be avoided. 



The number of scintillations recorded under each of the 

 counts was separately corrected for decay of the source from 

 the time of removal from the emanation to the time the 

 count was made. The corrected counts without the target 

 were averaged and subtracted from the average of the 

 counts with the target; and except where contamination was 

 present, this figure was taken as the number of particles 

 deflected by the target. 



The Results. 



The use of slanting source and target made it impossible 

 to compute with any certainty the number of particles 

 deflected, on Darwin's theory. The theory could be tested, 

 however, from the fact that with elements such as Ag, Au, 

 and Pb, where the nuclear charge is high, the impacts 

 should not permit of a close enough approach of the nuclei to 

 cause the abnormal deflexions observed by Prof. Rutherford 

 for light atoms. The ratio of the number of particles deflected 

 by copper, silver, gold, and lead, according to Darwin's 

 theory, eomputedfor a target and source normal to the path 

 of the particles, to the number actually observed should be 

 about constant, while the ratio for light elements should 

 •be abnormally high, if close encounters take place. 



The average results of a series of counts on Pb, Au, Ag. 

 Cu, S, s«nd Al are given in column () of Table I. for 10 mg. 

 of RaO. In column 2 we have the number calculated 

 from Darwin's theory for source and target normal to the 

 -original direction oi: projection of the particles, for 105° for 

 10 mg. RaC, and for 1 cm. air equivalent of deflecting 

 substance using the arrangements in the diagram. In 

 •column 3 the thickness of the deflecting layer is given as 

 calculated from the range of the particles after deflexion 



