48 STIMULATION OF THE HUMAN RETINA IN SITU. HARRIS. 



known of currents caused in this waj^ being sufficiently intense 

 to give pain in the upper gold-covered tooth when that tooth 

 had an unduly sensitive nerve. I therefore suggest that 

 the electric current thus produced was conducted through 

 the bones and tissues of the head and, encountering the 

 retina en route, stimulated it to give rise to the subjective 

 sensations of light. The possibility of stimulating the 

 retina in situ by electric current was discovered by Ritter 

 in 1800; a constant current passed either transversely across 

 the head in the temporal regions or from the eyelid to the 

 neck will, both at make and break, stimulate the retina 

 causing flashes of light to be perceived. I renewed my 

 acquaintance with these effects by passing the constant 

 current from one dry cell through the eyes transversely across 

 the head; at the make and break the flashes due to this 

 current were slightly less vivid than the flashes due to the 

 tooth-current. On using two dry cells, I obtained flashes 

 closely resembling those from the tooth-current. I am 

 assured that the current from two dry cells would be painful 

 to the inflamed nerve of a tooth. 



It is well known that pressure on the eye-ball produces the 

 sensation of light — the phosphene. I noticed that the 

 subjective flashes from two dry cells were not quite so vivid 

 as the phosphene from moderate (non-painful) pressure 

 on the eye-ball. The electrically produced flashes are more 

 diffuse than the phosphene which has a circular outline; the 

 sensations from drj^ cells or the tooth-current are more 

 truly flashes. 



Based on these considerations, I make an attempt to 

 estimate the amount of the tooth-current, say, during the first 

 few seconds of its production. Assuming that the resistance 

 of the head is about 3000 ohms, and that each dry cell can 

 develop 1.5 volts; then, for two cells, wc have 



1.5 X 2 1 



^ "^ Qonr P "^ ^,^ of an ampere or one milliampere. 



