Visual Sensations caused by a Magnetic Field. 189 



observers, and also with the physical condition of the observer 

 during the test. The observer rapidly becomes fatigued, and 

 after the excitation has continued for a while, larger values 

 are required to produce noticeable effects." 



" (6) Increasing the ampere-turns also increases the 

 intensity of the light sensations." 



' ; (c) The frequency of alternations has a marked effect on 

 the visual sensations. With the same strength of field the 

 effect appears greatest between 20 and 30 cycles per second." 



" At frequencies below 15 per second the light pulsates 

 in a succession of flashes over the whole field. From 20 to 

 35 cycles the light appears as a network of standing waves 

 on which is superimposed a quivering flickering effect. The 

 size of the meshes is smaller for the higher frequency. 

 Above 40 cycles the light becomes more uniform and the 

 flicker more rapid. ' ; 



" (d) The effect is the greatest in the temporal parts of 

 the field. In this connexion it is interesting to recall 

 Schoen's experiments upon the sensitivity to light intensities 

 of the nasal and temporal halves of the retina, in which he 

 found that the nasal retina was more sensitive than the 

 temporal retina/' 



The cause of this phenomenon appears from fig. 10 (q.v.). 

 The nasal halves of the retina? are cut by more lines of force 

 than are the temporal halves of the retinaa. 



The purpose of this paper is to present the results of 

 experiments which were aimed at the solution of a single 

 problem, viz., the dependence of the threshold of the light 

 sensation upon the frequency of the current. Knowing the 

 relatively simple law which W. Nernst and his pupils found 

 to exist between the minimal effective stimuli of alternating 

 currents of high frequency and intensity of current, we 

 hoped to discover a similar simple relationship between 

 frequency of alternation of the magnetic field and the 

 minimal effective intensity of current just sufficient to cause 



the sensation of light. Nernst's law: — K= — r=* where K 



V7 

 is a constant, t the minimal intensity of current just sufficient 

 to excite a motor nerve such as n. ischiadicus of the frog, 

 and / the frequency of the current, holds good for a large 

 number of results. It applies, as Nernst has shown, not 

 only to motor, but also to sensory nerves. If the mode of 

 excitation of a nerve by the alternating magnetic field is 



* Pfliiger's Archiv, vol. cxxii. p. 293 (1908). 



