416 "N THE THhXWY OK COMPOUND COLOURS. 



not merely good eyes, but a power of judging as to the exact nature of the 

 difference between two very pale and nearly identical tints, whether they differ 

 in the amount of red, green, or blue, or in brightness of illumination. 



In the following paper I shall first lay down the mathematical theory of 

 Newton's diagram, with its relation to Young's theory of the colour-sensation. 

 I shall then describe the experimental method of mixing the colours of the 

 spectrum, and determining the wave-lengths of the colours mixed. The results 

 of my experiments will then be given, and the chromatic relations of the 

 spectrum exhibited in a system of colour-equations, in Newton's diagram, and 

 in three curves of intensity, as in Brewster's diagram. The differences between 

 the results of two observers will then be discussed, shewing on what they 

 depend, and in what way such differences may affect the vision of persons 

 otherwise free from defects of sight. 



III. Mathematical Tlieory of Newton's Diagram of Colours. 



Newton's diagram is a plane figure, designed to exhibit the relations of 

 colours to each other. 



Every point in the diagram represents a colour, simple or compound, and 

 we may conceive the diagram itself so painted, that every colour is found at 

 its corresponding point. Any colour, differing only in quantity of illumination 

 from one of the colours of the diagram, is referred to it as a unit, and is 

 measured by the ratio of the illumination of the given colour to that of the 

 corresponding colour in the diagram. In this way the quantity of a colour is 

 estimated. The resultant of mixing any two colours of the diagram is found 

 by dividing the line joining them inversely as the quantity of each ; then, if 

 the sum of these quantities is unity, the resultant will have the illumination 

 as well as the colour of the point so found ; but if the sum of the components 

 is different from unity, the (fuantity of the resultant will be measured by the 

 sum of the components. 



This method of determining the position of the resultant colour is mathe- 

 matically identical with that of finding the centre of gravity of two weights, 

 and placing a weight equal to their sum at the point so found. We shall 

 therefore speak of the resultant tint an the sum of its components placed at 

 their centre of gravity. 





