NERVE 157 



(3) By allowing difterent parts of the spectrum to fall 

 upon the eye at the same time, it is possible to pro- 

 duce either a sensation of white or of some other 

 part of the spectrum {Practical Physiology). To pro- 

 duce a sensation of white from two or three difterent 

 parts of the spectrum a due proportion of each part must 

 be taken, since diff'erent parts have difterent sensational 

 activity. 



This effect of mixing different parts of the spectrum 

 means that by different 'modes of stimulation of the retina 

 the sarfie sensation may he produced. The sensation of 

 orange may be produced, either when vibrations at about 

 5 80 billions per second fall on the eye, or when two 

 sets of vibrations, one about 640 and one about 560 

 billions, reach it. By no possible physical combination of 

 the two is it possible to produce the intermediate rate of 

 vibration. 



The sensation of colour, therefore, depends upon the nature 

 of the change set up in the retina, and not upon the condition 

 producing that change. 



What we call colours are particular changes in conscious- 

 ness accompanying particular changes in brain neurons pro- 

 duced by particular changes set up in the retina, in whatever 

 way these changes may have been produced. 



Colour-blindness. — While every one is colour-blind at the 

 periphery of the retina, a certain proportion of people — 

 about 5 per cent. — are unable to distinguish reds and greens, 

 even at the centre of the retina. Colour-blindness for yellow 

 and blue is very rare. 



In complete loss of colour vision — monochromatic vision 

 — everything appears grey, and usually there is blindness in 

 the middle of the field over the central spot. Possiblv it is 

 produced by a loss of function in the cones. 



It is not known whether a condition of colour-blindness 

 exists in lower animals. 



When white light passes through a lens it is partly 

 decomposed into its component parts, the blue rays being 

 more refracted than the red, and chromatic aberration, the 

 unequal focussing of the difterent parts of the spectrum on 



