450 Proceedings of Eoyal Society of Edinhurgli. [july 16 , 
red. What Briicke saw was possibly the middle stage of what I 
have described above ; but his explanation can hardly be the correct 
one, because the reddish phase is got with a higher speed than the 
green. 
Altogether it seems scarcely to admit of doubt that the curves 
do follow different courses in their ascent. 
2 . How must the curves be drawn in order to explain our 
phenomena ? 
According to Kunkel, the first part of each of the three curves is 
a straight line. It is evident that if the three curves started from 
the same point, and continued as straight lines for the whole of 
their course, there could be no difference in colour corresponding to 
difference in length of stimulation. The condition that the curves 
should all start from the same point, is the same as that the three 
M 
Fig. 2 . 
sets of fibres should be all excited by the shortest stimulus used. 
Let the three straight lines O^Aj , O2A2 , O3A3 (fig. 2 ), represent 
the curves for red, green, and blue respectively; time being 
measured along the horizontal axis, and intensity of excitation cor- 
responding to any given time of stimulation along the vertical axis. 
To avoid confusion, the three curves have been drawn to separate 
axes, but the points Oj , O2 , O3 , where they start from the abscissa 
line, all lie on the same common perpendicular. For time of stimu- 
lation Oj , B^ , the stimulus being white light, let us suppose that the 
resultant impression has a certain colour, say a reddish tinge. Then 
the colour is determined by the proportion A^B^ : A2B2 : A3B3. 
