408 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



capable of direct verification. To put the matter in a nut-shell, 

 the older conception of partial colour-blindness as due to the 

 absence of a normal component is simple and, if true, practical, 

 but is not, in fact, quite adequate ; the more general form is 

 adequate, but not very helpful. 



Next, what has the Component Hypothesis to say with regard 

 to the phenomena of after-images ? 



A noteworthy feature of after-image experiments is, of course, 

 that stimulation with a given light increases responsiveness to its 

 complementary. It would appear, therefore, easy to imagine that 

 activity of the three components, or any one or two of them, in 

 a certain way diminishes their responsiveness in one direction, 

 increasing it in an opposite direction. This amounts to supposing 

 that we have a condition comparable with the state of the reflex 

 arcs, so brilliantly described by Sherrington : the nervous path is 

 occupied by one form of motor discharge, and this very occupancy 

 paves the way for a discharge different, and even opposite in kind. 



To so highly general a statement as this, no objection will be 

 found, but if we investigate details, difficulties arise. For instance, 

 the apparent saturation of spectral colours is greatly enhanced by 

 previously stimulating the eye with their complementaries. Helm- 

 holtz accordingly supposed that all the spectral colours stimulate 

 each visual component. But, if this be true, the simpler inter- 

 pretation of colour-blindness once more fails. Observations of 

 dichromatics suggest that lights having wave-lengths greater than 

 550 HIJL do not affect the third component (the " blue " or 

 " violet " component) at all, because no standard blue had to be 

 mixed with the standard red in order to effect a good match. To 

 a normal eye, however, the saturation of spectral yellow (589 /AJU) 

 is unquestionably enhanced by previous exposure to blue. Either 

 spectral colours do not affect all three components, in which case 

 the theory does not cover after-image effects, or the simpler 

 explanation of partial colour-blindness must be abandoned. In 

 view of what has already been said, the reader will perhaps agree 

 that the second alternative is the more plausible, and conclude that 

 after-images are adequately described at the cost of strengthening 

 our suspicion that dichromatic and trichromatic systems cannot be 

 co-ordinated in any simple manner. 



So far we have found that the Hypothesis of Three Components 

 describes with sufficient clearness the facts of normal colour vision, 



