ON THE THEORY OF THREE PRIMARY COLOURS. 447 



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 vibration. 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 infini- 

 tesimal 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 sensation 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, therefore, 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. 



