Sensations caused by a Magnetic Field, 



195 



Inasmuch as the series of ten observations did not require 

 ordinarily more than ten minutes, the exigencies of fresh air 

 could safely be met. In determining the threshold, the field 



was begun subliminally and gradually increased until the 

 observer signalled, by means of an electric light which he 

 operated with a button, that the visual sensation was just 

 apparent. Although the field was gradually increased in 

 strength by taking out resistance, the field was intermittently 

 made and broken by a switch which was operated by an 

 assistant. A pendulum of suitable length enabled the 

 assistant to keep the switch closed for one second and open 

 for an equal length of time. The period of stimulation and 

 the intervening interval were thus kept tolerably constant. 

 Following the observation in which the stimulus was in- 

 creased from subliminal to liminal, there occurred an 

 interval of a few seconds when the instruments were read 

 and recorded. Inasmuch as three persons co-operated in 

 this part of the experiment, the work was done rapidly. In 

 the second observation of the series, the stimulus was begun 

 far above the threshold and decreased slowly and inter- 

 mittently, as described above, until the sensation of light 



02 



