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XXIV. Visual Sensations caused by a Magnetic Field, 

 By 0. E. Magnusson and H. C. Stevens*. 



[Plates III. & IV.] 



IN a previously published paper t the authors verified and 

 extended the observations of S.P. Thompson J, Dunlap§, 

 nnd others on the visual sensations caused by a magnetic 

 field. It was shown that the effective condition of the 

 stimulus is the number of ampere-turns in the coil, and not 

 a current of high amperage as was implied in the work of 

 Dunlap. It was further shown that the magnetic field pro- 

 duced by a direct current at the moment of making or 

 breaking had a similar effect to that produced by an alter- 

 nating current. The chief results of our former work are 

 here briefly summarized for the convenience of the reader. 



Summary for Direct Currents. 



" (a) Visual sensations were produced by an increasing or 

 decreasing field when the circuit was closed or opened. No 

 effect was noticed when the current had reached a constant 

 value. 



" (b) The effect appeared as a narrow wave or band of 

 light in a horizontal plane and rapidly moving downwards 

 across the field of vision when the circuit is closed, and 

 upwards when the circuit is broken. 



" (c) The direction of the lines of force in the field (up or 

 down) did not affect the direction of motion of the light 

 wave. 



" (d) The wave observed on closing the circuit was brighter 

 and more definite in outline than the corresponding wave 

 when the circuit was broken. The rate of increase of field 

 strength on closing the circuit was also greater than the 

 corresponding decrease on opening the circuit. This tends 

 to prove that the intensity of the visual sensations depends 

 on the intensity and rate of change in the magnetic field." 



Summary for Alternating Currents. 



" (a) With the coil used, the threshold for current at 

 60 cycles was between 3000 and 4000 ampere-turns." 



" These values, no doubt, vary considerably with different 



* Communicated by the Authors. 



t Am. Journal Physiol, vol. xxix. p. 124 (1911). 



X Proceedings of the Royal Society, B. lxxxii. {567) p. 396. 



§ Science, n. s. vol. xxxiii. p. 68 (1911). 



