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.