and (2,1) reduces to 



e2 = |A tanh ^ (2.3) 



which is the equation for the velocity of any wave. Hence, if 



2ir T-" 



the surface tension can be reduced to such a magnitude that ~ --- 



is so small compared to gA/^-w that it can be neglected, then 

 the ripples would behave like waves. 



In order to obtain this situation, we have used Tergitol 

 Wetting Agent 7 to reduce the surface tension to the level where 

 (2.3) can be applied to a good degree of accuracy. The wetting 

 agent causes the surface tension/density ratio to be reduced to 

 about ,317 times that of water, which, for our purpose, is suffi- 

 cient. 



The photographic technique formerly employed, whereby the 

 "freezing" of the field by a strobolume enabled contact prints to 

 be made, has been discarded in favor of direct camera photography. 

 The advantages of this method are twofold: (1) it is not necessary 

 to operate under conditions of absolute darkness, and (2) the 

 whole wave field can be photographed. (In contact photography, 

 only the portion of the field covered by the photo-sensitive 

 paper can be "captured.") 



Instead of the strobe light used in earlier experiments, the 

 point source of light is now furnished by the bulb of a common 

 slide projector. The light is focussed by the wave crests on a 

 screen of ground glass, at a suitable height above the water surface, 

 so that the wave pattern is reflected as light and dark bands in 

 the mirror mounted above the ground glass screen. A schematic 



