the Photography of Ripples. 293 



In figs. 1 and 3, Paper 1, the series of hyperbolas due to 

 the interference of waves from two point-sources are shown ; 

 while in fig. 4, Paper 1, the hyperbolas are replaced by 

 another series of curves, the latter being due to the inter- 

 ference of waves of different length emanating from two 

 point-sources. When the distance between the sources is a 

 large number of wave-lengths, and the frequency of the two 

 sets of waves is the same, we get a series of ellipses shown in 

 fig. 2, Paper 1. 



In the present photograph the change which these ellipses 

 undergo when the sources emit waves of different length is 

 shown. The ellipses are now represented by a family of club- 

 shaped curves, with the blunt end pointing to the source of 

 higher frequency. 



The extreme case is seen in fig. 5, Paper 1. 



Figs. 4, 5, 6. Frequency 190. 



These photographs are intended to illustrate the fact that 

 in order to get wide interference- bands in optics it is necessary 

 to have the sources near together. The sources vibrate 

 in opposite phase ; one style being attached to the upper and 

 the other to the lower prong of the fork. It is a matter of 

 great importance to make the styles so rigid as to have a 

 natural frequency many times as great as that of the fork. 

 The style from the upper prong gave most trouble; it will 

 be noticed that it is a light braced structure of fine glass 

 rods. 



The line bisecting at right angles the distance between the 

 sources is always a line of minimum disturbance and is 

 represented on the photographs by a bright band. The 

 number of such lines passing between the sources is necessarily 

 odd. As the distance of the styles from each other decreases 

 the number of lines decreases, until finally, in fig. 6, we have 

 only the central band left. 



Fig. 6 at once recalls the experiment in acoustics in 

 which two organ-pipes of the same pitch are mounted 

 on one wind-chest. The distance between the styles 

 would have to be decreased to illustrate this experiment 

 with ripples. It would also be advisable to use longer 

 waves. 



It is not to be expected that a bright straight band drawn 

 through the sources would make its appearance with any 

 great distinctness even when the distance between the latter 

 was an even number of wave-lengths. This is owing to two 

 causes. Each source protects half of such a band from the 

 ripples due to the other ; and the amplitude of the vibration 



Phil. Mag. S. 5. Vol. 46. No. 280. Sept. 1898. Y 



