Effect caused by Incident and Emergent Light. 863 



be observed that the ratio has a constant value in each period. 

 One cannot, however, feel certain that this is a universal law, 



6. 



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ATOMIC WEIGHT. 



2.00 2i0 £4 



since it has only been possible to test it in the case of two 

 groups with two elements in each group. Moreover the 

 ratio of the emergent to the incident effect never differs 

 much from unity and there is no very great difference in the 

 values of the atomic weights of the elements in a given 

 group. Thus the range of possible variation is rather 

 restricted. 



Since it is known that the incident photo-electric effect 

 increases with decreasing wave-length of the incident light, 

 it may be that the ratio of the emergent to the incident effect 

 would be constant for monochromatic light, so that the 

 present variations in the above ratio of emergent to incident 

 effect for the various metals might be attributed to a selective 

 photo-electric ^ activity on part of the films u«ed. For 

 instance, platinum might respond photo-electrically more 

 easily to shorter wave-lengths than magnesium and hence 

 give a greater value for the ratio of the emergent to the 

 incident effect. At any rate this is a possibility which 

 should be carefully investigated. For this reason the inves- 

 tigation will be continued with the object of determining the 

 value of the above ratios for variations with the wave-length 

 of the incident light. 



From the existence of similar effects to those here described, 

 in the case of the 7-rays and Rontgen rays, Bragg has con- 

 cluded that these radiations are of a corpuscular character. 



