Accidental Electrical Stimulation of the Human 

 Retina in Situ. — By D. Fraser Harrjs, M. D., 

 D. Sc, F. R. S. E., Professor of Physiology in Dal- 

 housie University, Halifax. 



(Read 10 May 1915) 



In the summer of 1912 I had a lower left molar tooth filled 

 with a temporary stopping consisting of an amalgam of at 

 least three metals, silver, mercury, and tin. Within half 

 an hour of having this inserted I noticed that each time I 

 clenched the jaws at all forcibly, there appeared a bright 

 flash of light in the left eye; all through the rest of the day 

 flashes of light, getting fainter and fainter, kept recurring. 

 I noticed that the tooth in the upper jaw which touched the 

 amalgam in the lower was gold-capped. The light exper- 

 ienced was of a canary yellow and more like the sensation of 

 a vivid lightning flash (forked lightning) than of any mere 

 luminosity or diffusion of light. So vivid were these sub- 

 jective flashes that my first thought was that there had 

 been lightning, but on the day in question there was no 

 lightning, thunder or rain. 



The first explanation that occurred to me was that this 

 was a case of "contact electricity" (Kelvin), that the dis- 

 similar metals — the gold of the upper tooth and the amalgam 

 of the lower — having been in contact in the liquid saliva, pro- 

 duced sufficient current to stimulate the retina heterologously. 

 But the following experiment seemed to be against this; I 

 held between the teeth a copper rod in contact with a silver 

 rod, but no flash of light was experienced when these metals 

 were pressed together. 



The next explanation which occurred to me was that the 

 crystallization of the amalgam could give rise to currents 

 sufficiently intense to be the cause of physiological stimula- 

 tion. Dr. Frank Woodbury kindly told me that he had 



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