Lamp and gas-lights throw out yellowish-coloured rays, causing a great many light colours to appear different in tone from what they really are. Certain shades of green and blue are not easily distinguished by gaslight. A blue fabric will appear to be green, or of a greenish tone, caused by the yellow rays falling on it. Green being formed by the mixture of blue and yellow, whatever contributes yellow to blue, as in the case cited, or by mixture of pigments, the hue will be green. When coloured rays fall on a coloured surface, which is lighted by diffused daylight, the coloured surface is changed, the effect being the same as that produced by adding to it a pigment of the same colour as the coloured light. When red rays fall upon a black stuff they make it appear of a purple black; on white stuff they make it appear red; yellow stuff they make appear orange; and light blue stuff they make appear violet. _Complementary Colours._ “The colour required with another colour to form white light is called the complementary of that colour. Thus green is the complementary of red, and _vice versâ_; blue is the complementary of orange, and _vice versâ_; yellow is the complementary of violet, and _vice versâ_, because blue and orange, red and green, yellow and violet, each make up the full complement of rays necessary to form white light.” These remarks are deduced from experiments with a prism of glass, giving the spectrum or analysis of the coloured rays forming white light. When a ray of sunlight is passed through a triangular prism of flint glass, and the image received on white paper, it will be noticed that the spectrum (as the image is termed) consists of several colours—seven in all. Red, yellow, and blue are the most prominent. The red rays are modified by the pure yellow, and we have orange; the yellow rays becoming mixed with the blue become green; the blue and red give violet. Look intently for a few moments at a bright red object, then suddenly transfer the gaze to a sheet of white paper; the paper will appear of a greenish tint. Reverse the process; look intently at green, then on white paper, and red will be the tone of the paper. Blue will excite the eye to see orange, and orange will excite the eye to see blue. This is called successive contrast. In placing colours near each other it is of the greatest importance that the painter should bear in mind the foregoing laws. From these laws the coach-painter may derive some useful hints. In ornamenting and striping bear in mind that colours that are complementary purify each other. The effect of placing white near a coloured body is to heighten that colour. Black placed near a colour tends to lower the tone of it. Grey increases the brilliancy and purity of the primary colours, and forms harmonies with red, orange, yellow, and light green. _Chiaro-oscuro and Flat Tints._