414 Mr. J. H. Vincent on the 



which was electrically excited. The two forks, placed ap- 

 proximately parallel, are tied together by a piece of thread 

 about two feet long, so that the thread is at right angles to 

 both forks. This thread is then adjusted to an appropriate 

 tension by trial, when the maintained fork causes the other 

 to vibrate for any length of time. This simple device, which 

 has been used in the Royal College of Science laboratory for 

 some time, is due to Mr. W. Watson. 



Description of Photographs. 



The plates used were of various kinds, and any of the well- 

 known plates gave good results. Some of the negatives were 

 intensified previous to printing. The figures are about 

 \ natural size. My best thanks are due to Mr. R. Chapman, 

 who has assisted me throughout with great zeal. 



Photographs showing a series of circular waves set up by 

 a single style attached to a fork of known frequency were 

 taken with a view to quantitative measurements of surface- 

 tension, wave-length, and velocity. A bar of wood was so 

 placed that two needle-points which it carried nearly touched 

 the surface of the mercury. These points are a known dis- 

 tance apart, and by measuring the negatives we may find the 

 scale of reduction of lengths along this line. The particulars 

 of the motion are obtained from the equations 



llT Ap 



from which we find ; in a particular experiment, 

 n= 180 per second, 

 X=*165 centim., 

 v = 29*7 centim. a second, 

 T = 306 dynes per linear centim. 



This low value of the surface-tension was obtained from 

 mercury which had stood in the apparatus for some days. 

 The values obtained in a similar way previously were 420, 

 421, 365. The tension falls as the mercury gets more con- 

 taminated. The value of the surface-tension of pure dry 

 mercury is usually quoted at 540. 



Fig. 1. Two styles are attached to the same prong of a fork, 

 the frequency of which is 120. One centre is unfortunately 

 hidden by the fork. The approximately straight dark lines 

 which are seen to radiate from the region between the centres 

 of oscillation are lines of minimum disturbance; they are 

 hyperbolas of which the centres of disturbance are the foci. 

 This photograph illustrates the interference phenomena in 

 Optics produced by Young's or FresnePs methods. 



Fig. 2. The frequency of the fork is 256. Both styles are 



