HELMHOLTZ IN KONIGSBERG 



of a saturated spectral colour (or purple) of a deter- 

 minate tone. From the physical point of view, mixed 

 light is compounded of various waves of different wave- 

 lengths ; but the sensation caused by the mixture falling 

 on the retina may be always considered as a function of 

 three variable quantities (i) the quantity of saturated 

 coloured light ; (2) the quantity of white light which 

 may be added to produce the same sensation of 

 colour; and (3) the length of the wave of coloured 

 light. He also investigated mathematically the con- 

 struction of a geometrical table of colours. Finally, 

 he revived and extended the hypothesis of Thomas 

 Young, 1 which attempted to explain and account for 

 the phenomena of colour. 



How comes it that we perceive differences in 

 colour ? \Ve may look for the cause in various 

 directions. We might suppose a molecular vibration 

 to be set up in the nerve-endings synchronous with 

 the undulations of the luminiferous ether, without 

 any change in the chemical constitution of the sensory 

 surface ; and we might suppose that where various 

 series of waves corresponding to different colours 

 act together, these are fused together, or interfere 

 with each other in such a way as to give a vibration 

 of modified form or rate corresponding somehow to 

 the sensation arising in consciousness. Or, again, 

 we might suppose that the effect of different-coloured 

 rays is to promote or retard chemical changes in the 



1 Lectures on Natural Philosophy, 1 807. 

 I2 3 



