The fatigue of the retina to colour after it has been presented to the eye for any length of time is a difficulty, but in a less degree. That the retina does experience fatigue can be shown by a very simple experiment. The lecture theatre is now illuminated by the incandescent light, and if we throw an image of the bright carbon points of the electric arc light on the screen and steadily fix the eyes on the image of the white-hot crater for some (say) twenty seconds, and then we suddenly withdraw it, a _dark_ image of the points will be seen on the partially lighted screen, and will appear to travel with the eyes as they move away from the fixed point. This phenomenon is due to the fact that the perceiving apparatus for white light gets fatigued on the parts of the retina on which the bright image of the white carbon points thrown on the screen fell, and that when the source of brightness was removed, the less intense illumination of the screen failed to stimulate the vision apparatus at those parts to the same extent that they were stimulated over the rest of the field. We can vary the experiment by placing a red glass in front of the electric light, and, following the same course as before, we shall see a greenish-blue image of the carbon points upon the screen. In this case the retinal apparatus which has not been stimulated by the red sensation will be capable of the maximum stimulation by the feeble white light, whilst that part which has suffered fatigue will not respond so freely to the red contained in the white light. If we abstract a certain amount of red from the spectrum, its recombination will give a white tinged with greenish blue, which is a counterpart of the colour we feel when the eyes have been fatigued by the red light. CHAPTER III. Let me take you back again to matches of colour. We will now, however, make the matches with the primary colours in the guise of pigments. These colours themselves are complex colours, but as the eye cannot trace any difference, or at all events very little difference, between them and simple colours, a mixture of these complex colours should answer nearly as well as do mixtures of the simpler colours. We have here three discs, a red, a green, and a blue, and we can very closely match these colours by a red, a green, and a blue in the spectrum.