As the wave of light with its alternating electric fields comes up to the prism, the field will first displace the electrons in one direction and then in the other, and so on. If the period of one particular type of electron happens to coincide with the period of the wave, that electron will vibrate violently and will in its turn send out a series of waves in the glass. If the wave is an irregular one it will start all the electrons vibrating, but those electrons will vibrate most violently whose periods are equal to the periods of the Fourier's constituents which have the greatest energy. Thus we shall actually have the Fourier's constituent waves separated into the vibrations of different electrons. But the speed with which any simple wave travels in glass or in any transparent medium, other than a vacuum, is dependent upon its period. The shorter the period, _i.e._ the shorter the wave-length, the slower is the speed in most transparent substances. But the slower the speed in the prism the more is the ray deviated, and therefore we conclude that the violet end of the spectrum consists of the shortest waves while the red end consists of the {32} longest waves, and that the different parts of the spectrum are simple waves of different period. +The Whole Spectrum.+--The visible spectrum is by no means the whole of the series of Fourier's waves, however. The eye is sensitive only to a very small range of period, while there exists in sunlight a range many times as great. Those waves of shorter period than the violet end of the visible spectrum will be deviated even more than the violet, and will therefore be beyond the violet. They are called the ultra violet rays, and can easily be detected by means of their chemical activity. They cause a number of substances to glow, and therefore by coating the screen on which the spectrum is received with one of these substances, the violet end of the spectrum is extended by this glow. The waves of longer period than the red rays will be deviated less than the red, and will therefore lie beyond the red end of the visible spectrum. They are called the infra-red rays, and are chiefly remarkable for their heating effect.