196 Messrs. Magnusson and Stevens on Visual 



became visible, the observer signalling with his light when 

 this point was reached. 



By thus alternating, a series of ten observations were 

 taken with their corresponding readings on the instruments. 

 In Table I. are shown the intensities of current in the coil 

 for each observer's ten observations for each frequency. 

 These intensities are the values of the stimulus which were 

 just sufficient to cause a sensation. 



The abbreviations "Inc." and " Dec." stand for increasing 

 and decreasing, respectively. An increasing stimulus is one 

 which changes from subliminal to liminal. A decreasing 

 stimulus is one which changes from supraliminal to liminal. 

 Also the average (Av.), the mean variation (M.V.), and the 

 probable error (p. e.) of the ten readings are given. H' is 

 calculated by the formula 



H' = K x i, , (8) 



where K is the constant derived from fl^. 5 and i the average 

 threshold value of the current. 



Observed Data and Tables. 



The ammeter readings for threshold values are recorded 

 in Table I. The value of J37 is the product of the average 

 current and the constants obtained from fig. 5. 



Table II. and curve in fig. 11 give the maximum strength 

 of field H' in lines per sq. cm. along the axis of the coil and 

 in the plane of the optic nerve. 



Table III. and curve in fig. 12 give fx H', values pro- 

 portional to the induced voltages. Assuming that the 

 sensation of light is caused by this induced electromotive 

 force, then the numbers in Table III. represent the relative 

 magnitude of the stimuli producing threshold effects for the 

 several frequencies. 



If a second assumption be made, that in whatever nerve 

 circuits or paths this alternating E.M.F. may be induced,, 

 there exists a simple resistance only, then the resulting 

 current should also be proportional to the same series. 

 Hence the quantities in Table III. correspond to the term 

 " i " in Nernst's law (1) . Dividing these values by y/f we 

 obtain Table IV. If the above assumptions be correct, and 

 since the values given in Table IV. are not constant but 

 increase with the frequency, it is shown that Nernst's law 

 does not apply to this form of nerve-excitation. 



