18 Britain's Heritage of Science 



implying action at a distance, those who were guided by 

 Newton considered it to be almost a sacrilege to go further 

 than the master. To them action at a distance became 

 an universal dogma, and the undulatory theory had no chance 

 until it could produce a conspicuous success by explaining 

 experimental facts, which were not amenable to treatment 

 by the more favoured hypothesis. 



The analogy of light to sound attracted the attention 

 of Thomas Young (1773-1829), and was emphasized by 

 him in a paper published in the Philosophical Transactions 

 of the Royal Society in 1800. Here, again, it was the 

 detailed examination of one special aspect of the problem 

 which led to the decisive advance. Some of the charac- 

 teristic features of a wave motion may be illustrated by 

 an examination of the waves passing over a sheet of water. 

 Everyone is familiar with the circles spreading out from 

 a centre when a stone is thrown into water; each point 

 of the surface as the wave passes over it rising and falling 

 alternately. If two stones are thrown, and enter the water 

 at points near each other, each will start its own system 

 of circles. These will overlap, and the question arises : 

 how does the motion at any point of the surface of the 

 water depend on the motion due to each wave separately? 

 The question is so simple, and the answer seems so easy, 

 that many must have passed it by as hardly worth 

 recording; but Young saw that it was the key to the 

 position : each wave produces its own effect without inter- 

 ference from the other. If, under the influence of one set 

 of waves, a point were raised one inch above the undisturbed 

 level, and the other set caused by itself alone an elevation 

 of two inches, then the combined effect would be a rise of 

 three inches. If the effect of the second wave at any time 

 were a depression of two inches, the effect of the first being 

 the same as before, the depression of two inches would 

 overbalance the rise of one inch, and leave a depression 

 amounting to one inch. If the rise due to one set of 

 waves equals exactly the fall due to the other, there will 

 be neither a rise nor a fall, but the point will remain 

 at rest. This, in a few words, is the principle of " super- 

 position of motions," which applies only approximately to 



