There are several well-known methods of mixing colours, such as the superposition of two spectra or of different parts of the same spectrum—the method of reflection, Czermak's modification of Schemer's experiment, the use of rotating disks having coloured sectors, and the direct mixture of coloured powders or coloured liquids. In all of these cases the effect may be seen with one eye, and is due to the action of light on a definite portion of one retina. But may sensations of mixed colours be produced by binocular vision of the components ? Regarding this question various well-known observers have arrived at completely opposite results.