372 Professor J. C. Maxwell [May 17, 



eye, fall on a sensitive membrane, and we become aware of various 

 colours. We know that the colour we see depends on the nature of 

 the light ; but the opticians say there are an infinite number of kinds of 

 light ; while the painters, and all who pay attention to what they see, 

 tell us that they can account for all actual colours by supposing them 

 mixtures of three primary colours. 



The speaker then next drew attention to the physiological difficulties 

 in accounting for the perception of colour. Some have supposed that 

 the different kinds of light are distinguished by the time of their vibra- 

 tion. There are about 447 billions of vibrations of red light in a 

 second ; and 577 billions of vibrations of green light in the same time. 

 It is certainly not by any mental process of which we are conscious 

 that we distinguish between these infinitesimal portions of time, and it 

 is difficult to conceive any mechanism by which the vibrations could be 

 counted so that we should become conscious of the results, especially 

 when many rays of different periods of vibration act on the same part 

 of the eye at once. 



Besides, all the evidence we have on the nature of nervous action 

 goes to prove that whatever be the nature of the agent which excites a 

 nerve, the sensation will differ only in being more or less acute. By 

 acting on a nerve in various ways, we may produce the faintest sensa- 

 tion or the most violent pain ; but if the intensity of the sensation is 

 the same, its quality must be the same. 



Now, we may perceive by our eyes a faint red light which may be 

 made stronger and stronger till our eyes are dazzled. We may then 

 perform the same experiment with a green light or a blue light. We 

 shall thus see that our sensation of colour may differ in other ways, 

 besides in being stronger or fainter. The sensation of colour, there- 

 fore, cannot be due to one nerve only. 



The speaker then proceeded to state the theory of Dr. Thomas 

 Young, as the only theory which completely reconciles these difficulties 

 in accounting for the perception of colour. 



Young supposes that the eye is provided with three distinct sets of 

 nervous fibres, each set extending over the whole sensitive surface of 

 the eye. Each of these three systems of nerves, when excited, gives 

 us a different sensation. One of them, which gives us the sensation we 

 call red, is excited most by the red rays, but also by the orange and 

 yellow, and slightly by the violet ; another is acted on by the green 

 rays, but also by the orange and yellow and part of the blue ; while the 

 third is acted on by the blue and violet rays. 



If we could excite one of these sets of nerves without acting on 

 the others, we should have the pure sensation corresponding to that 

 set of nerves. This would be truly a primary colour, whether the 

 nerve were excited by pure or by compound light, or even by the action 

 of pressure or disease. 



If such experiments could be made, we should be able to see the 

 primary colours separately, and to describe their appearance by re- 

 ference to the scale of colours in the spectrum. 



