70 



SCIENCE 



[N. S. Vol. XXXIII. No. 837 



carried out under the supervision of Mr. 

 Henry 0. Louis of the company's electrical 

 engineering department. 



We forced 440 amperes of 60 cycle alterna- 

 ting current through the coil, with terminal 

 potential drop of 32 volts. The flicker now 

 became much more distinct than it had been 

 at the lower amperage. Dr. Whitehead, Dr. 

 Jennings and one of the students who before 

 had not noticed any flicker, were present at 

 these tests and now got the flicker very clearly 

 at first trial. Mr. Louis and a number of the 

 electricians also testified to the unmistakable 

 nature of the phenomenon. 



With 480 amperes of 25-cycle current (20 

 volts) a much more striking result was ob- 

 tained. With my head below the level of the 

 coil, and with my eyes open, the flicker was 

 strongly noticeable, although the room was 

 brightly lighted by afternoon daylight. The 

 whole visual field quivered as if illuminated 

 by a rapidly intermittent light. Several other 

 subjects made a similar observation, although 

 in some cases the flicker was noticed only in 

 the less illuminated parts of the visual field, 

 as where shadows fell in the room. With the 

 head inside the coil the flicker was so pro- 

 nounced as to be intensely disagreeable. The 

 flicker seemed to me slower than with the 60- 

 cycle current, and Mr. Louis and one of his 

 assistants found the same apparent difference. 

 Others were uncertain as to this point. The 

 flicker with 60-eycle current had seemed to me 

 to differ in character from ordinary visual 

 flicker; it was odd, or novel in an indefinite 

 way; but the flicker with 25-cycle current 

 seemed quite like ordinary rapid flicker. This 

 difference I am inclined to explain by the fact 

 that normally a flicker of 60 per second is 

 imperceptible, hence such a flicker seems un- 

 usual when produced under these abnormal 

 conditions. 



With the 200-ampere current, I had found 

 that with the head either above or below the 

 coil, with the face turned either upward or 

 downward, practically no effect was obtained, 

 although in two of these positions the eyes 

 were close to the plane of the end of the coil, 

 and hence in a strong magnetic field. Rota- 



tion of the head from one of these positions 

 through 90° (presenting the side of the head 

 to the coil) brought in the flicker distinctly. 

 My observation on this point was confirmed 

 by Dr. Anderson and Dr. Watson. With the 

 stronger currents the effect was much more 

 pronounced. Although some flicker was ob- 

 served when the occipito-frontal axis was 

 vertical, rotation of the head through 90° 

 caused a great increase. This would suggest 

 that the effect is due to induction currents in 

 the optic pathway, since in the position with 

 occipito-frontal axis vertical, the general di- 

 rection of the optic pathway is parallel to the 

 lines of force, whereas rotation of the head 

 through ninety degrees brings the pathway 

 across the lines. When the head is inside the 

 coil, the pathway crosses the lines in the most 

 intense part of the magnetic field. 



Whether currents induced in the optic 

 pathway excite the occipital cortex directly, 

 or excite the retina primarily, is yet a matter 

 for conjecture. That flicker is produced by 

 alternations faster than the fastest flicker 

 from normal light stimulation is of course no 

 evidence for the non-retinal character of the 

 flicker in question. 



I can not say as yet that there is a definite 

 arousal of visual sensation by the alternating 

 field; the effect appears more like an alternate 

 intensification and inhibition of whatever 

 sensory process is already in progress. That 

 is to say: if a certain intensity of normal 

 light-sensation or idio-retinal light is present 

 before the current is turned on, the apparent 

 effect of setting up the alternating field is 

 alternately to increase and decrease the in- 

 tensity of the sensation so that the average 

 intensity is not changed. It is quite possible 

 that further observation will change my opin- 

 ion on this point. 



No sensations other than the visual, which 

 could be connected with the alternating field 

 were noticed by any of us. That there is no 

 after-effect from the stronger fields, I should 

 not like to say at present. I should advise any 

 experimenter to proceed cautiously. 



It is very desirable that experiments with a 



