596 



SCIENCE 



[N. S. Vol. XL. No. 1034 



Fig. 1. 



tinctly marked, and persisted for hours. The 

 bright spot at the opposite pole from the sun 

 was only occasionally visible. The rainbows 

 were brilliantly colored and could be seen 

 until the sun was almost down. 



The sky at the time was almost clear, except 

 for a few wisps of cloud and a thin haze which 

 was densest directly over the face of the sun. 

 A. W. Freeman 



SCIENTIFIC SOCKS 

 Photo-electricity. By H. Stanley Allen. 



London, Longmans, Green and Co., 1913. 



8vo. Pp. ix-f221. Price $2.10 net. 

 Photo-electricity. By Arthur Llewelyn 



Hughes. Cambridge, The University Press, 



1914. 8vo. Pp. viii + 144. 



The present generation of physicists has 

 seen the rapid and almost spectacular devel- 

 opment of several important fields of activity 

 in physics : such, for example, as the subject 

 of electric waves, of kathode rays and elec- 

 trons, of X-rays, and of radioactivity. While 

 the subject of photo-electricity has not aroused 

 the same widespread and popular interest as 

 the subjects just mentioned, there are at 

 present many reasons for believing that the 

 study of photo-electric phenomena may prove 

 to be of almost equal importance in its bear- 

 ing upon theories of atomic structure and of 

 radiation. 



I imagine that most physicists have read 

 the paper in which Hertz described his dis- 

 covery of the photo-electric effect. The paper 

 is reprinted in Hertz's "Ausbreitung der 

 elektrischen Kraft " and in the English trans- 



lation " Electric "Waves." I can think of no 

 scientific article which illustrates so well not 

 only what research in experimental physics 

 ought to be, but also how the results should 

 be presented. It is a good illustration also 

 of the importance of the unexpected things 

 that so frequently turn up in experimental 

 work. It will be remembered that the dis- 

 covery of the photo-electric effect came as an 

 incident in Hertz's work on electric waves. 

 As difficulty was experienced in seeing the 

 minute sparks that indicated the response of 

 the resonator, he tried to improve matters by 

 placing a box around the gap so as to screen 

 the eyes. But instead of making it easier to 

 see the sparks the box apparently made the 

 resonator less sensitive. I imagine that most 

 of us would have been content to call the 

 attempted improvement a failure, and would 

 have dismissed the matter with mingled feel- 

 ings of mild wonder that the scheme didn't 

 work, and regret that we had wasted so much 

 time in malting the box. But Hertz was not 

 content to simply wonder. He set out to dis- 

 cover why the box had such an unexpected 

 effect, and by a beautifully logical series of 

 experiments and deductions he found the an- 

 swer to his question. Since it appeared that 

 the new phenomenon had no bearing upon 

 what he regarded as his more important prob- 

 lem, he left its further study to others and 

 returned to the subject of electric waves. 



Hertz's paper aroused wide-spread interest 

 and the work was quickly taken up by others. 

 During the first nine years after Hertz's dis- 

 covery more than one hundred articles deal- 

 ing with the photo-electric effect were pub- 

 lished, and interest in the subject has con- 

 tinued undiminished since. As no resume of 

 the subject has been published which is at all 

 complete, it is clear that the physicist who 

 wishes to make himself familiar with what has 

 been done in this important field has no small 

 task before him.^ The almost simultaneous 



1 A rgsumS of work on. the photo-electric effect 

 was published in Science, Vol. IV., p. 853 and p. 

 890, 189&, which was, I believe, complete to the 

 time of publication. The subject has developed 

 so greatly since that time, however, that this sum- 

 mary has little more than historical interest. 



