the Colour of the Sky. 



429 



the third by violet * rays. The impression of colour is the 

 resultant of the intensities of these three effects. The impres- 

 sion produced by any kind of light is always the sum of the 

 effects its components are capable of producing upon the three 

 sets of nerve-ends. Helmholtz discovered that the impression 

 upon these nerves is not directly proportional to the intensity 

 of the ray, but that the " red "-nerves are subject to one law, 

 and the " green " to another, while the " violet "-nerves have 

 a distinct law of their own. For very feeble rays, the " violet "- 

 nerves are very sensitive, while the "green" and "red"- 

 nerves scarcely act at allf . As the intensity of the light in- 

 creases, the "red" and "green "-nerves increase in activity, 

 while the " violet "-nerves gradually become tired and dazzled, 

 as it were, and so incapable of receiving their full share of the 

 effect. For rays of dazzling brilliancy the " red "-nerves are in 

 their most sensitive condition, while those affected by the more 

 refrangible components of the ray are comparatively dull %. 

 The comparative effect of yellow and blue light at different 

 intensities can be shown by curves (see figure). For feeble 



TStlow. 



--Mite 



° Intensity of Itrty 



intensities, the blue impression exceeds the yellow ; the point 

 soon comes where the curves cut one another ; beyond this 

 yellow exceeds the blue, and the latter seems to be approach- 

 ing a maximum. Upon this peculiarity of the eye depend all 



* The colours first chosen by Young were red, yellow, blue— probably 

 because these were the commonly accepted " primaries." Later he adopted 

 red, green, violet— a choice confirmed since by Helmholtz, Maxwell, &c. 



t The expressions " red "-nerve, " green "-nerve, &c. are used here for 

 the sake of brevity to denote the nerve-termini affected by those colours. 



\ Helmholtz, Handbuch derphysiologische Optik, p, 319 j see also Dobro- 

 lowsky, Graef s Archiv der Opthalmologie, Bd. xviii. 



