388 Lord Rayleigh on the Tension of Water 



Faraday are especially to be avoided as almost necessarily 

 of large amplitude. At the same time the limitation is not 

 without its inconvenience- One of the great difficulties of the 

 experiment is to see the waves properly, and this is much 

 increased when the vibrations are extremely small. 



In considering the problem thus presented, it occurred to 

 me that it was essentially the same as that so successfully 

 solved by Foucault in relation to the figuring of optical sur- 

 faces. The undisturbed surface of liquid is an accurate plane; 

 the waves upon it may be regarded as deviations from optical 

 truth, and may be made evident in the same way as any other 

 deviations from truth in a reflecting surface. Guided by this 

 idea, I was able to work with waves of which nothing what- 

 ever was to be seen by ordinary observation of the surface 

 over which they were travelling. 



In the application of Foucault's method it is necessary that 

 light from a radiant point, after reflexion from the surface 

 under test, should be brought to a focus, in the immediate 

 neighbourhood of which is placed the eye of the observer. 

 Any small irregularities in the surface then render themselves 

 conspicuous to the eye focused upon it. In the present case 

 the reflector is plane, and the formation of a real image of the 

 radiant requires the aid of a lens. In my experiments this 

 was usually a large single lens of 6 inches diameter and 

 84 inches focus. On one occasion an achromatic telescope- 

 lens was substituted, but the aperture was too small to include 

 the number of waves necessary for accuracy. Although the 

 want of achromatism was prejudicial to the appearance of the 

 image, it is not certain that the accuracy of the determina- 

 tions was impaired, at least after experience in observation had 

 been acquired. The lens was fixed horizontally near the floor, 

 a few inches above the surface of the water under examination. 

 The radiant point, a very small gas-flame, was situated in the 

 principal focal plane, but a little on one side of the axis of 

 symmetry, so that the image formed after reflexion from the 

 water and a double passage through the lens might be a little 

 separated from the source. For greater convenience reflecting 

 strips of looking-glass were introduced at angles of 45°, or 

 thereabouts, so that the initial and final directions of the rays 

 were horizontal. 



The smallness of the disturbance is not the only obstacle to 

 its visibility. Even with Foucault's arrangement for viewing 

 minute departures from planeness, nothing could usually be 

 seen of the waves here employed without a further device 

 necessary on account of the rapidity with which all phases are 

 presented in succession. A clear view of the waves must be 



