+The Spectrum. Dispersion.+--When a narrow beam of white light is transmitted through a prism of glass or of any other transparent substance, it is deflected from its original direction and is at the same time spread out into a small fan of rays instead of remaining a single ray. If a screen is placed in the path of these rays a coloured band is formed on it, the least deflected part of the band being red and the colours ranging from red through orange, yellow, green, blue, and indigo, to violet at the most deflected end of the band. This band of colours is called the spectrum of the white light used, and the spreading out of the rays is called dispersion. +Newton's Experiment.+--Newton first discovered this fact with an arrangement like that in Fig. 17. [Illustration: FIG. 17.] If by any means the fan of coloured rays be combined again into a single beam, white light is reformed, and Newton therefore came to the conclusion that white light was a mixture of the various colours in the spectrum, and that the only function of the prism was to separate the constituents. Of the nature of the constituents Newton had little knowledge, since he had rejected the wave theory, which could alone give the clue. {30} We now believe that white light is an irregular wave, and that the prism manufactures from it the Fourier's series of waves to which it is equivalent. It is supposed that the manufacture is effected by means of the principle of resonance. As an example of resonance let a small tap be given to a pendulum just as it commences each swing. Then because the taps are so timed that each of them increases the swing of the pendulum by a small amount, they will very soon cause the pendulum to swing very violently even though the effect of a single tap can scarcely be detected at all. Thus when any body which has a free period of vibration is subject to periodic impulses of the same period as its own, it will vibrate very vigorously and absorb nearly all the energy of the impulses. +Electrons and their Vibrations.+--There is conclusive evidence to show that in the atoms of all substances, and therefore of the glass of which the prism is composed, there are a number of minute negatively electrified particles which are called electrons. These are held in position by a positive charge on the rest of the {31} atom, and if they are displaced from their usual positions by any means they will vibrate about these positions. The time of vibration of the electron will depend upon its position in the atom and upon the position of neighbouring atoms. In solid or liquid bodies the neighbouring atoms are so near that they have a considerable influence in modifying the period of an electron or a system of electrons, and consequently we may find almost any period of vibration in one or other of these electrons or systems.