184 HERMANN VON HELMHOLTZ 



lecture at Carlsbad on ' After - Images ' (September, 1858), 

 part of which had already been given to the Nieder-Rhein- 

 ische Gesellschaft. He pointed out in the first place that 

 the Theory of the Three Fundamental Colours could not be 

 retained, in the sense of deriving all actual objective colours 

 from any given three such objective colours, since it is im- 

 possible, if we select any three spectral colours (as the most 

 saturated colour we know), to derive all remaining spectral 

 colours from them, as the resulting mixture is always more 

 or less white. Young's theory is, however, independent of this, 

 when it states that there are three principal colour sensa- 

 tions, distributed to three systems of nerve fibres, which 

 can be excited collectively, but in different degrees of in- 

 tensity, by all kinds of light, so that they yield qualitatively 

 different sensations ; here the choice of fundamental colours 

 is arbitrary, to a certain extent. In any case spectral colours 

 will not excite the separate fundamental colour sensations pure, 

 and distinct from the other two; this would agree with the 

 view of Helmholtz set forth in his Theory of After-images, 

 that there are more highly saturated sensations of colour than 

 those which are aroused by spectral colours. In support of 

 Young's hypothesis, Helmholtz examined a colour-blind subject 

 with the help of Clerk Maxwell's colour-tops (which in sound 

 eyes produce any given colour by the mixture of three 

 suitable fundamental colours, with the addition of white, 

 exhibited on sectors of variable breadth), and found Maxwell's 

 results confirmed, since his patient could match all colours 

 by mixtures of yellow and blue; thus for his colour-blind 

 eyes one of the fundamental sensations was wanting. He 

 found the colours which the colour-blind confuse with 

 neutral grey to be red and green-blue, the red of which 

 appeared to them dark-grey, and the complementary greenish- 

 blue a very light grey, since the colour-blind eye was found 

 to be very insensitive to red. By this means red was proved 

 to be one of the fundamental colours. Helmholtz gives 

 the name of red-blindness to this kind of colour-blindness, in 

 which, according to Young's theory, there is a paralysis of the 

 red-perceiving sensory nerves. He regarded it as probable 

 that the other class hitherto denoted as colour-blind are green- 

 blind, although the experiments had not at that time been 



