+Wave Theory. Rectilinear Propagation.+--True, Huyghens in 1678 had propounded the theory that light consists of waves of some sort starting out from the luminous body, and he had shown how readily it expressed a number of the observed facts; but light travels in straight lines, or appears to do so, and waves bend round corners and no one at that time was able to explain the discrepancy. Thus for nearly a century the theory which was to be universally accepted remained lifeless and discredited. The answer of the wave theory to the objection now is, that light does bend round corners though only slightly and that the smallness of the bend is quite simply due to the extreme shortness of the light waves. The longer waves are, the more they bend round corners. This can be noticed in any harbour with a tortuous entrance, for the small choppy waves are practically all cut off whereas a considerable amount of the long swell manages to get into the harbour. +Interference of Light. Illustration by Ripples+.--The revival of the wave theory dates from the discovery by Dr. Young of the phenomenon of interference of light. In order to understand this we will {14} consider the same effect in the ripples on the surface of mercury. A tuning-fork, T (Fig. 5), has two small styles, S S, placed a little distance apart and dipping into the mercury contained in a large shallow trough. When the tuning-fork is set into vibration, the two styles will move up and down in the mercury at exactly the same time and each will start a system of ripples exactly similar to the other. At any instant each system will be a series of concentric circles with its centre at the style, and the crests of the ripples will be at equal distance from each other with the troughs half-way between the crests. [Illustration: FIG. 5.] The ripples from one style will cross those from the other, and a curious pattern, something like that in Fig. 6, will be formed on the mercury. S S represents the position of the two styles, while the plain circles denote the positions of the crests and the dotted circles the positions of the troughs at any instant. Where two plain circles cross it is evident that both systems of ripples are producing a crest, and so the two produce an exaggerated crest. Similarly where two dotted circles cross an exaggerated trough is produced. Thus in the shaded portions of the diagram we get more violent ripples than those due to a single style. Where a plain circle cuts a dotted one, however, one system of ripples produces a {15} crest and the other a trough, and between them the mercury is neither depressed below nor raised above its normal level. At these points, therefore, the effect of one series of ripples is just neutralised by the effect of the other and no ripples are produced at all. This occurs in the unshaded regions of the diagram. The mutual destruction of the effects of the two sets of waves is "Interference." [Illustration: FIG. 6.]