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LVI. On the Photography of Ripples. By J. H. Vincent, 

 B.Sc, A.R.C.Sc, Assistant Demonstrator in Physics at the 

 Royal College of Science, London, S. W* 



[Plates I.-1IL] 



MANY of the phenomena described in this paper have 

 been exhibited at public lectures by Mr. C. Y. Boys, 

 using the stroboscopic method, which was first applied to the 

 study of ripples by Lord Rayleigh. I am indebted to Mr. 

 Boys for having recommended to me the work of photograph- 

 ing these effects, and also for many valuable suggestions. 



Lord Kelvin defines a ripple as a wave whose length is 

 less than that of the wave which is propagated with the 

 minimum velocity. For ordinary mercury, waves less than 

 1*3 centim. long are ripples. Capillary ripples are those 

 whose length is so small as to render negligible, in the value 

 of the velocity squared, the term due to gravity. These 

 definitions are rendered clearer by reference to Mr. Boys's 

 Logarithmic Wave Chart. The portion of the curve, repre- 

 senting the relation of the velocity and wave-length, to the 

 left of the point of minimum velocity, refers to ripples. The 

 straight-line portion to the left of the chart represents capil- 

 lary ripples. 



In order to obtain ripples it is necessary to use vibration- 

 frequencies above a certain value. Thus in the case of ordi- 

 nary clean-looking mercury, with its damp and probably 

 greasy surface, the surface-tension of which may be between 

 300 and 400 C.G.S. units, a frequency of about 15 per second 

 causes the biggest waves which the above definitions include 

 as ripples ; while frequencies of about 200 and upwards give 

 rise to waves whose propagation is practically controlled by 

 surface-tension, and these waves are capillary ripples. 



Now the duration of the sensation produced by a luminous 

 impression on the retina lasts for about one eighth of a second; 

 thus we are unable to see ripples on the surface of mercury. 

 The frequencies employed are generally many times the maxi- 

 mum visible frequency. It is not the high velocity of propa- 

 gation which renders ripples invisible ; ripples produced by a 

 disturbance of a frequency of about 200 do not travel very 

 quickly, a foot a second being about the order of magnitude 

 of the velocity on the surface of ordinary mercury. 



* Communicated by the Physical Society : read Feb. 26, 1897. 



