§ 1. Introductory.—Having given a general account of the nature of sensation, and of the sense-reaction, we now pass to the special senses, beginning with those we know most about, sight and hearing. Sight is a vehicle of spatial perception, and it is so in part because of the peculiar nature of visual sensation. But we shall postpone treatment of this part of the subject, until we come to deal with perceptual as distinct from sensational consciousness. At present we are only concerned with the peculiar modifications of consciousness specifically corresponding to differences in the nature of the physical stimulus which we call light. In other words, we have to deal with coloursensations, including the neutral tints, white, black, and intermediate greys.

§ 2. Nature of the Stimulus.—Physically considered, light is an undulating movement of the particles of a generally diffused medium called the luminiferous ether. For our purposes, we may represent this undulating movement by the waves which pass along a rope, when it is fixed at one end, and jerked up and down by the hand at the other. As the wave traverses the rope, what travels along it is not of course the material particles of the rope themselves, but only a form of movement which is transmitted from one set of particles to another. The hand may move more or less quickly; the more quickly it moves, the shorter are the waves. In the undulating movement the particles of the rope first rise above and then fall beneath their position of equilibrium when the rope is at rest. They rise to a crest, and sink into a hollow. The length of the wave is measured by the distance between the point at which this movement begins and the point at which it terminates. Longer waves traverse the rope in the same time as shorter ones; hence the shorter wave must be more frequently repeated in the same time. Thus the shorter the wave the shorter time it takes to complete itself. The amplitude of the wave must be carefully distinguished from its length. The hand, while continuing to repeat its movements in the same time, and consequently producing waves of the same length, may take a more or less extended swing. The more extended the swing, the greater is the amplitude of the waves that traverse the rope. The particles of the rope rise higher and sink lower; their crests are higher and their hollows deeper. Suppose now that the hand, in making its excursion to and fro, also trembles. Two different kinds of impulse are then communicated to the rope, each of which separately would give rise to waves of different length. The result is waves of a more complex form which can be mathematically explained as due to a combination of the waves which the separate impulses would severally produce.

Thus we can distinguish three characteristics of an undulating movement: (1) wavelength, (2) amplitude, (3) simplicity or complexity. In the case of light, each of these characters of the physical undulation is specially connected with a corresponding characteristic of visual sensation. Differences of wavelength are specially connected with differences of colourquality other than those which are constituted by degrees of paleness or darkness, viz. by more or less resemblance to white or black. Colourquality in this restricted sense is called colourtone. For example, the difference between yellow and green, or between yellowgreen and a still yellower green, is a difference of colourtone. The difference between yellow and yellowishbrown is difference in saturation due to a darkening of the yellow. The amplitude of the wave is specially connected with the intensity of the sensation. Any specific colourtone, such as green or red, produced by light of a certain wavelength, may be made brighter or less bright by increasing or diminishing the intensity of the light, viz. the amplitude of the vibration. It may become brighter without alteration of its colourtone. If we have a series of greys including what we call white, arranged in a graduated scale of brightness, it is possible to fix the brightness of a given colour, such as green, by comparing it with the greys. It is judged to be equally bright with one of them, and more or less bright than the rest. The complexity of a wave determines what is called the degree of saturation or purity of the corresponding colour. We can, as we have seen, compare a green with a grey or white in respect of intensity or brightness: but we can also compare it in another respect: we can ask how far the green resembles the grey in quality. It may be a greenish grey or a greyish green, or apparently a pure green. The more it approximates to grey, the less saturated it is, and the more free it is from any apparent admixture of grey, the more saturated it is.

It must not be supposed that colourtone is determined solely by wavelength, intensity solely by amplitude, and degree of saturation solely by complexity. It is only within certain limits that the physical intensity of light can be varied without affecting colourtone. Variation in the intensity of the light also affects saturation; increase makes the colour whiter, and decrease makes it darker. Wavelength not only determines colourtone, but also helps to determine brightness. Some colourtones are brighter than others, even though the physical stimulus is less intense. Complexity of vibration is a very important factor indeed in determining colourtone. The same colours which are produced by simple waves can be produced by complex waves also, though in a less pure or saturated form. White or grey results from a combination of lights of all wavelengths, and also from various other combinations. In ordinary daylight, all wavelengths are combined.