extended series of observations with it. Maxwell's color top consists of a heavy disk with perpendicular spindle. Sectors of different colored papers can be placed on the disk, and made to overlap more or less; a smaller colored disk can be attached so as to cover the central part only. When the top is made to spin, the reflected colors which succeed one another in position are mixed in the eye, and the mind perceives a uniform color. The angular lengths are adjusted till, if possible, a match is made with the color in the centre; then the color equation is read off.
As regards the electrical line of investigation he had already conceived the idea of making the old mathematical theory of electrical attraction and repulsion, as elaborated by Coulomb and Poisson, harmonize with the method by which Faraday was obtaining splendid results, namely, the consideration of the lines of force in the medium. With this end in view he studied the German and French writers; and in the winter of 1855-56 he published a paper on Faraday's lines of force.
At the age of 24 he gained, after competitive examination, a fellowship from his college. Soon after, the chair of physics (natural philosophy it is there called) in Marischel College, one of the teaching colleges of the University of Aberdeen, Scotland, fell vacant; and Maxwell was advised by his old friend Forbes to become a candidate for the appointment. The suggestion agreed with his own aims as to a career, and he found that his father also approved of it. He sent in his application; and was appointed but not before his father had died. So, in the spring of 1856 he became both the master of Glenlair and the professor of physics in Marischel College, Aberdeen University.
He entered on his teaching work at Aberdeen with great enthusiasm. A professor in the Scottish Universities is free to teach his subject according to the most approved method, and is not bound to bend all energies towards fitting his students for an examination conducted by independent examiners; this feature of his duties Maxwell valued highly. At Cambridge he had taken a share in lectures to workingmen, and at Aberdeen
