An independent investigator of this subject was Clerk Maxwell, who experimented with a “colour-box” of his own design, by which he mixed the simple colours of the spectrum, and the results he got are really the first which are founded on measurement. He measured something, but hardly arrived at the colour sensation. His colour-box took two forms, both on the same principles, so only one will be here described, the diagram and description being taken from his classic paper in the Philosophical Transactions of the Royal Society for 1860. “The experimental method which I have used consists in forming a combination of three colours belonging to different portions of the spectrum, the quantity of each being so adjusted that the mixture shall be white, and equal in intensity to a given white. Fig. 14 represents the instrument for making the observations. It consists of two tubes, or long boxes of deal, of rectangular section, joined together at an angle of about 100°. [Illustration: FIG. 14. Maxwell’s colour-box.] “The part A K is about five feet long, seven inches broad, and four deep; K N is about two feet long, five inches broad, and four deep; B D is a partition parallel to the side of the long box. The whole of the inside of the instrument is painted black, and the only openings are at the end A C, and at E. At the angle there is a lid, which is opened when the optical parts have to be adjusted or cleaned. “At E is a fine vertical slit, L is a lens; at P there are two equilateral prisms. The slit E, the lens L, and the prisms P are so adjusted, that when light is admitted at E, a pure spectrum is formed at A B, the extremity of the long box. A mirror at M is also adjusted so as to reflect the light from E, along the narrow compartment of the box to B C. “At A B is a rectangular frame of brass, having a rectangular aperture of six inches by one. On this frame are placed six brass sliders, X Y Z. Each of these carries a knife-edge of brass in the plane of the surface of the frame. “These six movable knife-edges form three slits, X Y Z, which may be so adjusted as to coincide with any three portions of the pure spectrum formed by light from E. The intervals behind the sliders are closed by hinged shutters, which allow the sliders to move without letting light pass between them. “The inner edge of the brass frame is graduated to twentieths of an inch, so that the position of any slit can be read off. The breadth of the slit is ascertained by means of a wedge-shaped piece of metal, six inches long, and tapering to a point from a width of half an inch. This is gently inserted into each slit, and the breadth is determined by the distance to which it enters, the divisions on the wedge corresponding to the 200th of an inch difference in breadth, so that the unit of breadth is ·005 inch.