Thus Niven, at the close of his obituary notice in the Proceedings of the Royal Society, says: “It is seldom that the faculties of invention and exposition, the attachment to physical science and capability of developing it mathematically, have been found existing in one mind to the same degree. It would, however, require powers somewhat akin to Maxwell’s own to describe the more delicate features of the works resulting from this combination, every one of which is stamped with the subtle but unmistakable impress of genius.” And again in the preface to Maxwell’s works, issued in 1890, he wrote: “Nor does it appear to the present editor that the time has yet arrived when the quickening influence of Maxwell’s mind on modern scientific thought can be duly estimated.”
It is, however, the object of the present series to attempt to give some account of the work of men of science of the last hundred years, and to show how each has contributed his share to our present stock of knowledge. This task, then, remains to be done. While attempting it I wish to express my indebtedness to others who have already written about Maxwell’s scientific work, especially to Mr. W. D. Niven, whose preface to the Maxwell papers has been so often referred to; to Mr. Garnett, the author of Part II. of the “Life of Maxwell,” which deals with his contributions to science; and to Professor Tait, who in Nature for February 5th, 1880, gave an account of Clerk Maxwell’s work, “necessarily brief, but sufficient to let even the non-mathematical reader see how very great were his contributions to modern science”—an account all the more interesting because, again to quote from Professor Tait, “I have been intimately acquainted with him since we were schoolboys together.”
Maxwell’s main contributions to science may be classified under three heads—“Colour Perception,” “Molecular Physics,” and “Electrical Theories.” In addition to these there were other papers of the highest interest and importance, such as the essay on “Saturn’s Rings,” the paper on the “Equilibrium of Elastic Solids,” and various memoirs on pure geometry and questions of mechanics, which would, if they stood alone, have secured for their author a distinguished position as a physicist and mathematician, but which are not the works by which his name will be mostly remembered.
The work on “Colour Perception” was begun at an early date. We have seen Maxwell while still at Edinburgh interested in the discussions about Hay’s theories.
His first published paper on the subject was a letter to Dr. G. Wilson, printed in the Transactions of the Royal Society of Arts for 1855; but he had been mixing colours by means of his top for some little time previously, and the results of these experiments are given in a paper entitled “Experiments on Colour,” communicated to the Royal Society of Edinburgh by Dr. Gregory, and printed in their Transactions, vol. xxi.
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.”