§ 13. =Sensations from the Eye.=—You may study tones by help of the piano and a few medicine bottles; but for the study of lights and colours you must go beyond household appliances, and secure a fairly large set of coloured and grey papers; sample-books may be obtained, very cheaply, from the manufacturers. You will notice, first of all, that as the world of sounds divides into tones and noises, so does the world of looks divide into what we have just called colours and lights. The colourless looks or =lights= may be arranged in a single straight line that passes from purest white through the greys to deepest black; they are, as sensations, older than colours, just as noise is older than tone. =Colours= are more varied. Consider, to begin with, the character of colour proper or =hue=, that is, the differences of colour that show in the rainbow. Hues may be arranged, not in one straight line, but in a square. Setting out, say, from red, you pass through red-yellow or orange to yellow; that is one straight line; setting out again from yellow, you pass through yellow-green to green; from green you pass through green-blue to blue; and finally from blue you come back, by way of blue-red (violet and purple), to the original red. Colours have, besides, two further characters, that bring them into relation with lights. They differ in =tint=, that is, in darkness or lightness; brown is darker than yellow, sky-blue is lighter than navy-blue. They differ also in saturation or =chroma=, that is, in poorness or richness of hue; pinks and yellows look faded and washed-out as compared with rich reds and blues. _Tint brings colours into relation with lights_, because, if we can say that a colour is darker or lighter than a particular grey, we can also find some grey that matches it in darkness or lightness; and _chroma brings colours into relation with lights_, in the sense that the better chroma is farther off from colourlessness (that is, from grey) than the poorer chroma of the same hue and tint. _All lights and colours are psychologically simple._ Paints may be mixed on a palette, and colour-stimuli may be mixed in all sorts of ways; we learn in physics that white daylight is a mixture of all the rays that are seen separately in the rainbow. Yet a white, considered just as a look, is perfectly simple; and the looks of orange and yellow-green and green-blue are equally simple. There are no compound colours, to correspond with compound tones. Hence the number of light and colour sensations is very large, at least ten times as large as the number of simple tones.