ON THE THEORY OF COMPOUND COLOURS. 411 



although less intense ; and all purples may be compounded of blue and red, 

 and diluted with white to any required tint. Brown colours, which at first 

 sight seem different, are merely red, orange or yellow of feeble intensity, more 

 or less diluted with white. 



It appears therefore that the result of any mixture of colours, however 

 complicated, may be defined by its relation to a certain small number of 

 well-known colours. Having selected our standard colours, and determined the 

 relations of a given colour to these, we have defined that colour completely as 

 to its appearance. Any colour which has the same relation to the standard 

 colours, will be identical in appearance, though its optical constitution, as 

 revealed by the prism, may be very different. 



We may express this by saying that two compound' colours may be chro- 

 matically identical, but optically different. The optical properties of light are 

 those which have reference to its origin and propagation through media, till it 

 falls on the sensitive organ of vision ; the chromatical properties of light are 

 those which have reference to its power of exciting certain sensations of colour, 

 perceived through the organ of vision. 



The investigation of the chromatic relations of the rays of the spectrum 

 must therefore be -founded upon observations of the apparent identity of com- 

 pound colours, as seen by an eye either of the normal or of some abnormal 

 type ; and the results to which the investigation leads must be regarded as 

 partaking of a physiological, as well as of a physical character, and as indicating 

 certain laws of sensation, depending on the constitution of the organ of vision, 

 which may be different in different individuals. We have to determine the 

 laws of the composition of colours in general, to reduce the number of standard 

 colours to the smallest possible, to discover, if we can, what they are, and to 

 ascertain the relation which the homogeneous light of different parts of the 

 spectrum bears to the standard colours. 



II. History of the Theory of Compound Colours. 



The foundation of the theory of the composition of colours was laid by 

 Newton*. He first shews that, by the mixture of homogeneal light, colours 

 may be produced which are "like to the colours of homogeneal light as to 

 the appearance of colour, but not as to the immutability of colour and consti- 



* Optics, Book i. Part 2, Props. 4, 5, 6. 



522 



