52 Mr. James Rice on 



simple matter of a ruler. It is nothing so easy. I should not 

 envy the man who essayed to measure the wave-length of a train 

 of moving ripples to an accuracy of, say, one per cent., with 

 nothing but a foot-rule for apparatus. And yet the measurement 

 has in it simplifying elements absent from the corresponding pro- 

 blem for light waves. The medium, water, appeals directly to 

 our senses ; its oscillations, which are of course not merely 

 surface phenomena, are all the more easily observed by the fact 

 that there is a surface whose changes of form disclose to us the 

 underlying movements. Further, even in the case of the tiniest 

 ripples, the wave-lengths are enormous compared with those of 

 light. However, if you look at this slide*, it may help you to 

 obtain a sufficient idea of the means adopted by the physicist for 

 attacking this problem. This picture is an instantaneous photo- 

 graph of the surface of a shallow dish of water which is being 

 disturbed by the oscillations of two needles, both vibrating Avith 

 equal periods, and each one exciting a ring-shaped train of ripples 

 whose wave lengths are equal. The apparatus employed in the 

 production of this picture is called a ripple-tank, and requires the 

 nicest adjustment between the mechanism actuating the needles, 

 which is an electrically driven tuning fork, and the photographic 

 apparatus, which is in reality a scientific adaptation of the cine- 

 matograph camera. The picture shows you an instantaneous 

 view of the surface of the water. What you observe is that along 

 a series of radial lines or channels, diverging from a spot between 

 the needles, the water is absolutely undisturbed, while along the 

 intervening radial channels you have the water surface corrugated 

 in crests and troughs, The photograph shows you that as a 

 matter of fact along certain paths there is no energy of vibration 

 transmitted at all ; the two wave-trains are said to interfere 

 along such lines. Along the intervening paths the wave-trains 

 reinforce one another, and all the energy emitted by the vibrating 

 mechanism is directed along these channels. The reason for this 



Fig. 1. 



