There is still another point to be cleared up--and it is one of the utmost importance as regards our present subject. Let O (fig. 17) be a spot in still water which, when disturbed, produces a series of circular waves: the disturbance necessary to produce these waves is simply an oscillation up and down of the water at O. Let _m_ _n_ be the position of the ridge of one of the waves at any moment, and _m'_ _n'_ its position a second or two afterwards. Now every particle of water, as the wave passes it, oscillates, as we have learned, up and down. If, then, this oscillation be a sufficient origin of wave-motion, each distinct particle of the wave _m_ _n_ ought to give birth, to a series of circular waves. This is the important point up to which I wish to lead you. Every particle of the wave _m_ _n_ _does_ act in this way. Taking each particle as a centre, and surrounding it by a circular wave with a radius equal to the distance between _m_ _n_ and _m'_ _n'_, the coalescence of all these little waves would build up the large ridge _m'_ _n'_ exactly as we find it built up in nature. Here, in fact, we resolve the wave-motion into its elements, and having succeeded in doing this we shall have no great difficulty in applying our knowledge to optical phenomena. [Illustration: Fig. 17.] Now let us return to our slit, and, for the sake of simplicity, we will first consider the case of monochromatic light. Conceive a series of waves of ether advancing from the first slit towards the second, and finally filling the second slit. When each wave passes through the latter it not only pursues its direct course to the retina, but diverges right and left, tending to throw into motion the entire mass of the ether behind the slit. In fact, as already explained, _every point of the wave which fills the slit is itself a centre of a new wave system which is transmitted in all directions through the ether behind the slit_. This is the celebrated principle of Huyghens: we have now to examine how these secondary waves act upon each other. [Illustration: Fig. 18.]