In the paper on “The Theory of Compound Colours,” printed in the Philosophical Transactions for 1860, Maxwell gives a history of the theory as it was known to him. He points out first the distinction between the _optical_ properties and the _chromatic_ properties of a beam of light. “The optical properties are those which have reference to its origin and propagation through media until it falls on the sensitive organ of vision;” they depend on the periods and amplitudes of the ether vibrations which compose the beam. “The chromatic properties are those which have reference to its power of exciting certain sensations of colour perceived through the organ of vision.” It is possible for two beams to be optically very different and chromatically alike. The converse is not true; two beams which are optically alike are also chromatically alike. The foundation of the theory of compound colours was laid by Newton. He first shewed 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 constitution of light.” Two beams which differ optically may yet be alike chromatically; it is possible by mixing red and yellow to obtain an orange colour chromatically similar to the orange of the spectrum, but optically different to that orange, for the compound orange can be analysed by a prism into its component red and yellow; the spectrum orange is incapable of further resolution. Newton also solves the following problem:-- _In a mixture of primary colours, the quantity and quality of each being given to know the colour of the compound_ (Optics, Book 1, Part 2, Prop. 6), and his solution is the following:--He arranges the seven colours of the spectrum round the circumference of a circle, the length occupied by each colour being proportional to the musical interval to which, in Newton’s views, the colour corresponded. At the centre of gravity of each of these arcs he supposes a weight placed proportional to the number of rays of the corresponding colour which enter into the mixture under consideration. The position of the centre of gravity of these weights indicates the nature of the resultant colour. A radius drawn through this centre of gravity points out the colour of the spectrum which it most resembles; the distance of the centre of gravity from the centre gives the fulness of the colour. The centre itself is white. Newton gives no proof of this rule; he merely says, “This rule I conceive to be accurate enough for practice, though not mathematically accurate.”