448 
Proceedings of Boyal Society of Edinhurgh, [july 16 , 
what happens in the central part. Here, as has been said, we get 
with increasing speed, first a rose-coloured or violet, then a green, 
and then a reddish tinge. With further increase of the speed no 
change is seen. The first thing to notice is that all those changes 
take place about dr below the speed necessary for complete and 
steady fusion of the separate flashes. The idea, therefore, at once 
suggests itself that the phenomena are connected with the different 
course of the curves representing the excitation of the three groups 
of fibres of the Young-Helmholtz theory. 
Tick says, in Hermann’s Handhucli der Physiologie, Bd. iii. s. 
220 — “ Das Anklingen der Erregung nach Beginn des Lichtreizes ist 
vom Standpunkte der Young’schen Theorie in jeder Fasergattung ein 
Vorgang fiir sich, und ist im Sinne dieser Theorie Keineswegs zu 
erwarten dass die drei gleichzeitigenYorgange genau gleichen Schritt 
halten. Ware der Gang des Anklingens in den roth-, griin-, und 
blau-empfindenden Fasern sehr verschieden, so miisste ein weisses 
Objekt in den ersten Momenten nach seinem Auftauchen im 
Gesichtsfelde gefarbt erscheinen. Davon nimmt man nun bei den 
Versuchen, weder nach meiner noch nach der Helmholtz’schen 
Methode, etwas entschiedenes wahr, woraus hervorgeht, dass der 
zeitliche Verlauf des Anklingens der Erregung bei Eeizung mit 
weiss aussehender Strahlung in den verschiedenen Fasergattungen 
annahernd derselbe ist.” 
By our method, if with a certain length of stimulation the 
excitement in one of the groups of fibres has an advantage over 
that in the others, all those small differences will be added together 
so as to produce a continuous effect, which may be kept up for any 
desired length of time. If it be granted, then, that the excitation 
curves do not follow precisely the same course — that is to say, differ 
in their time relations — it may at the outset be looked upon as pretty 
certain that, if the difference be at all considerable, it will be 
revealed by this method. And how could such a difference show 
itself except by colour changes of the kind described ? A certain 
proportion between the amount of excitement in the three fibre 
groups is associated with the sensation of white light. Disturb that 
proportion, and the light will be no longer white. Let the conditions 
be such as to favour the excitation of the red fibres in comparison 
with the others, and the light must ap]:>ear tinged with red. 
