126 PSYCHO-PHYSICAL METHOD 



were in their earliest infancy. Fechner had not then 

 published his Elemente der Psycho-physik. 



Helmholtz's results were accepted and no material 

 advance was made with the subject, in spite of the 

 work of one or two observers, until after fifty years 

 Professor Martius of Kiel arrived at a result very different 

 from Helmholtz's by means of a modification of his 

 method. The modification consisted essentially in this, 

 that instead of exhibiting two semicircles of light simul- 

 taneously for comparison, he exposed side by side two 

 circles of light of equal physical brightness. Of these 

 one was fixed and enduring, the other of brief duration 

 only and recurring at regular intervals of some seconds. 

 The observer had then to compare the brightness of 

 the momentarily appearing circle with that .of the en- 

 during circle, and the duration of the exposure of the 

 former was increased step by step until it appeared to 

 be equally bright with the latter. This duration was 

 then accepted as the action- time of light of that particular 

 intensity. And this was about one-tenth only of the 

 figure for the action-time found by Helmholtz's pro- 

 cedure. 



Was then this procedure faultless, and was Helmholtz's 

 result so very far wrong as Martius would have it ? 



Martius' procedure is as perfect as Helmholtz's physi- 

 cally, and it satisfies better the psychological conditions, 

 because the attention is turned directly from the one 

 impression to the other. Again, it avoids the error due 

 to contrast, but on the other hand a new flaw is intro- 

 duced into the physiological conditions of the experiment. 

 When a ray of light falls on the eye the sensation it 

 excites is brightest in the first moment of its appearance, 

 and declines very rapidly in brightness owing to fatigue 

 of the nervous substance, so that even within half a 



