412 ox THE THEORY OF COMPOUND COLOURS. 



tution of light- lied and yellow give an orange colour, which is chromatically 

 similar to the orange of the spectrum, but optically different, because it is 

 resolved into its component colours by a prism, while the orange of the spectrum 

 remains unchanged. When the colours to be mixed lie at a distance from one 

 another in the spectrum, the resultant appears paler than that intermediate 

 colour of the spectrum which it most resembles ; and when several are mixed, 

 the resultant may appear white. Newton* is always careful, however, not to 

 call any mixture white, unless it agrees with common white light in its optical 

 as well as its chromatical properties, and is a mixture of all the homogeneal 

 colours. The theory of compound colours is first presented in a mathematical 

 form in Prop. 6, "In a mii-tm-^ of prinuiry colours, tin- </iinntity and <jtiJiti/ 

 of each being given, to know the colour of the corn-pan ml." He divides the 

 circumference of a circle into seven parts, proportional to the seven musical 

 intervals, in accordance with his opinion about the proportions of the colours 

 in the spectrum. At the centre of gravity of each of these arcs he places a 

 little circle, whose area is proportional to the number of rays of the corre- 

 sponding colour which enter into the given mixture. The position of the centre 

 of gravity of all these circles indicates the nature of the resultant colour. A 

 radius drawn through it points out that colour of the spectrum which it most 

 resembles, and the distance from the centre determines the fulness of its colour. 



With respect to this construction, Newton says, " This rule I conceive 

 accurate enough for practice, though not mathematically accurate." He gives no 

 reasons for the different parts of his rule, but we shall find that his method 

 of finding the centre of gravity of the component colours is completely con- 

 firmed by my observations, and that it involves mathematically the theory of three 

 elements of colour ; but that the disposition of the colours on the circumference 

 of a circle was only a provisional arrangement, and that the true relations of 

 the colours of the spectrum can only be determined by direct observation. 



Young t appears to have originated the theory, that the three elements of 

 colour are determined as much by the constitution of the sense of sight as by 

 anything external to us. He conceives that three different sensations may be 

 excited by light, but that the proportion in which each of the three is excited 

 depends on the nature of the light. He conjectures that these primary sensa- 



7th and 8th Letters to Oldenburg. 



f Young'* Lectures on Natural Philosophy, Kelland's Edition, )>. 345, or Quarto, 1807, Vol. i. 

 p. 441 ; see alio Young in Philosophical Transactions, 1801, or Works in Quarto, VoL II. p. 617. 



