1888 .] Mr G. N. Stewart on Intermittent Light. 
449 
We have now to answer three questions : — 
1. What evidence is there that the excitation curves of the three 
hypothetical fibre groups, when time is taken as abscissa, follow 
different courses ? 
2. How must the curves be drawn to explain our phenomena 
granting that they have different courses 1 
3. Is it inconsistent with any known fact to draw the curves 
so? 
1. In regard to the first question, there is direct evidence that 
the curves which represent the decline of the excitement after 
stimulation has ceased are not parallel to one another for the three 
groups of fibres. Helmholtz, Fechner, and others have described 
a succession of colours in the positive after image of a white object, 
which seems quite analogous to that observed by me during the 
stimulation ; “ the positive after image goes quickly out of the 
original white through greenish-blue into indigo-blue, and then into 
violet or rose colour. Then follows a dirty orange, which is very 
often succeeded by a dirty yellowish-green ” (Hermann’s Handhuch 
der Physiologie, Bd. iii. s. 220, &c.). These after appearances can be 
very well explained on the supposition that the excitement falls 
away according to a different law in the three fibre groups ; and 
they scarcely seem capable of explanation without this supposition. 
Here we must assume “that the excitation in the elements which 
have to do with the sensation of red declines at first most quickly, 
then most slowly ; in the green elements at first most slowly, and 
then most quickly ; while in the blue the decline takes a middle 
course.” 
Is it probable, therefore, that if the curves have a different course 
in their decline they have also a different course in their rise. 
In the next place, Kunkel has investigated the time relations of 
stimulation with homogeneous light. He found that, when the 
different coloured rays are made of approximately equal physio- 
logical intensity, the red requires the shortest time to produce the 
full effect, and next to this the blue, while the green requires the 
longest time (Pfliiger’s Archiv, Bd. ix. s. 197, &c.). 
Briicke stated that white light is perceived as green when the 
successive flashes are very short, and he explained this by the sup- 
position that green requires a shorter time to excite the retina than 
