Januabt 13, 1911] 



SCIENCE 



69 



periment. Professor Whitehead readily agreed 

 to cooperate with me, and the first tests were 

 made in his laboratory and under his per- 

 sonal supervision. 



The coil used had 2Y turns of a cable con- 

 sisting of 3Y copper wires (each .082 inch in 

 diameter) equivalent to 250,000 circular mils. 

 The coil was approximately 8 inches long, 

 and elliptical in cross section, the internal 

 diameters being 10.5 inches and 9 inches. 

 With this coil no part of the subject's head or 

 face touched the internal surface; a condi- 

 tion which we could not always attain with a 

 coil of circular cross section and 9 inches in- 

 ternal diameter, which we first tried. The 

 coil was suspended from the ceiling by ropes, 

 so that the subject could sit in a chair with 

 his head inside the coil. The transformer 

 available forced approximately 200 amperes 

 of 60-cycle A.C. through the coil, the potential 

 drop between coil terminals being slightly 

 over 12 volts. This gave a field of 5,400 am- 

 pere turns, against 5,760 maximal in Thomp- 

 son's experiment. 



On first trial I distinctly perceived the 

 flicker. Dr. Anderson, Dr. Cowles, Dr. Essick 

 and a student also perceived it on first trial. 

 Dr. Watson was uncertain on first trial, but 

 on second trial perceived the flicker, although 

 not very distinctly until after several trials. 

 Dr. Jennings, Dr. Whitehead and several 

 others perceived absolutely nothing, even 

 after careful trials. It still seemed to me pos- 

 sible that idio-retinal light and suggestion 

 were at the bottom of the phenomenon and 

 therefore Dr. Watson and I carried out some 

 careful tests in which suggestion was ex- 

 cluded to the fullest possible extent, which 

 tests showed conclusively ■ that my suspicion 

 was unfounded. In these tests the trans- 

 former rested on a table close beside the coil, 

 so that the loud noise of the former com- 

 pletely drowned the hum of the latter. The 

 current could be switched off the coil, and 

 on resistance carrying practically the same 

 amperage as the coil, so that in either case the 

 transformer noise was the same. A tele- 

 phone receiver connected with the trans- 

 former was hung on the coil, emitting a 



loud noise whether the current was on the 

 coil or on the resistance. For further pre- 

 caution the subject's ears were plugged up as 

 well as was possible. Under these conditions, 

 where there was absolutely no way of telling 

 by the sound whether the current was on or 

 off the coil, each of us was able to identify the 

 flicker with absolute precision. 



Several subjects noticed a 'twitching of the 

 eyelids, when the head was in the coil. This 

 was noticed especially by two of the subjects 

 who were unable to perceive the flicker, and 

 who thought it probable that the muscular 

 sensations were at the base of the phenomenon. 

 They were asked to report on the twitching, 

 while the current was being turned now on the 

 coil, now on the resistance, and it was found 

 that the twitching occurred just as strongly 

 when there was no current on the coil as when 

 there was current. 



Those who perceived the flicker found it be- 

 coming less distinct after a minute or even 

 less of stimulation, and found it restored by 

 a few minutes' rest. 



The flicker was best perceived with the eyes 

 closed or with the room darkened, but was 

 noticeable with the eyes open if the room was 

 not too brightly lighted. The interior of the 

 coil furnished a fairly dark background. 



I made tests with both positive and nega- 

 tive after-images but could not find that the 

 flicker affected them in any way. The flicker 

 was strongest in the peripheral visual field, 

 and possibly did not affect to any considerable 

 degree the central portion of the field, in 

 which were the after-images. 



It was clear, as a result of these tests, that 

 the phenomenon was really a matter of visual 

 sensation, and that we were dealing with 

 threshold values; it needed only higher in- 

 tensity to make the flicker visible to all sub- 

 jects. Further it seemed to me strongly indi- 

 cated that current of less frequency would 

 give more intense flicker. Mr. A. E. Loizeau, 

 of the Consolidated Gas, Electric Light and 

 Power Company, kindly offered us the facili- 

 ties of his testing plant, and the coil was ac- 

 cordingly removed thither, and further tests 



