Let us now glance at some of the differences in quality recognized in sensation. First, we have the broadly distinguished groups of touches and pressures, temperature-sensations, tastes, smells, sounds, sights, muscular sensations, and organic sensations from internal parts of the body. And then, within each of these groups, there are the more or less delicate and distinct shades of quality, well exemplified in vision by the different colour-sensations, in hearing by notes of different pitch, and in smell by the varieties of scents and odours. Many of those sensations, moreover, which are apparently simple, are in reality compound. There are differences of quality in the note A as sounded on a violin, a piano, and a flute; and these differences are due to different admixtures of overtones, which fuse with the fundamental tone and alter its timbre. So, too, with vision. The sensation given by a white disc is a compound sensation, due to waves of different period, which separately would give sensations of colour. Sensations, then, differ in quality. They also differ in quantity or intensity. This needs little illustration. As evening falls, the sight-sensations derived from the surrounding objects grow more and more feeble. They may remain the same in quality, but the quantity or intensity gradually diminishes. So, too, in music, the pianos and fortes give us differences in intensity of sound-sensations. Sensations also differ in duration. The stimulation may be either prolonged or instantaneous. Two or more sensations may, moreover, be simultaneous or successive. Just as they may be either similar or different in quality and in intensity, so they may be either simultaneous or successive in time. Simultaneous sensations are best exemplified in vision and through touch; successive sensations are given most clearly by the sense of hearing, through which we recognize a sequence of sounds.