COLOUR VISION. 273 



this adjustment of illumination Young's triangle may be made to exhibit every 

 variety of colour. If we now take any two colours in the triangle and mix 

 them in any proportions, we shall find the resultant colour in the line joining 

 the component colours at the point corresponding to their centre of gravity. 



I have said nothing about the nature of the three primary sensations, or 

 what particular colours they most resemble. In order to lay down on paper 

 the relations between actual colours, it is not necessary to know what the 

 primary colours are. We may take any three colours, provisionally, as the 

 angles of a triangle, and determine the position of any other observed colour 

 with respect to these, so as to form a kind of chart of colours. 



Of all colours which we see, those excited by the different rays of the 

 prismatic spectrum have the greatest scientific importance. All light consists 

 either of some one kind of these rays, or of some combination of them. The 

 colours of all natural bodies are compounded of the colours of the spectrum. If 

 therefore we can form a chromatic chart of the spectrum, expressing the relations 

 between the colours of its different portions, then the colours of all natural 

 bodies will be found within a certain boundary on the chart defined by the 

 positions of the colours of the spectrum. 



But the chart of the spectrum will also help us to the knowledge of the 

 nature of the three primary sensations. Since every sensation is essentially a 

 positive thing, every compound colour-sensation must be within the triangle of 

 which the primary colours are the angles. In particular, the chart of the spec- 

 trum must be entirely within Young's triangle of colours, so that if any colour 

 in the spectrum is identical with one of the colour-sensations, the chart of the 

 spectrum must be in the form of a line having a sharp angle at the point 

 corresponding to this colour. 



I have already shewn you how we can make a mixture of any three of 

 the colours of the spectrum, and vary the colour of the mixture by altering 

 the intensity of any of the three components. If we place this compound 

 colour side by side with any other colour, we can alter the compound colour 

 till it appears exactly similar to the other. This can be done with the greatest 

 exactness when the resultant colour is nearly white. I have therefore constructed 

 an instrument which I may call a colour-box, for the purpose of making 

 matches between two colours. It can only be used by one observer at a time, 

 and it requires daylight, so I have not brought it with me to-night. It is 

 nothing but the realisation of the construction of one of Newton's propositions 



VOL. n. 35 



