Photography of Ripples. 415 



attached to the same prong. The photograph shows two series 

 of interference-curves, one a family of hyperbolas analogous 

 to those shown in fig. 1, and the other a family of ellipses. 



The hyperbolas are the radiating light lines seen on the 

 side of the photograph remote from the fork. They are fixed 

 in position, the little dark facets moving along between pairs 

 of hyperbolas. 



The light oval curves in the region between the centres of 

 disturbance are ellipses, since they are the loci of points of 

 intersection of the two series of circles whose radii grow 

 uniformly, and at the same rate with time. Their method 

 of production here is similar to a well-known geometrical 

 construction for ellipses. Unlike the system of hyperbolas, 

 these ellipses are not at rest. They travel outwards in such 

 a way that any ellipse occupies a position which was filled 

 previously by its predecessor a whole period before. That 

 semiaxis of any ellipse which passes through a centre of dis- 

 turbance grows with the same velocity as that with which 

 the ripples are propagated. The other semiaxis grows with a 

 velocity which is infinite at the commencement, but which 

 gradually decreases to the same uniform velocity of growth 

 as that of the first. The law of decrease of velocity is the 

 same as the law of decrease of the lengths of whole-period 

 elements of a linear wave with respect to a point. 



In order to render these ellipses stationary it would be 

 necessary to change one of the sources into a sink to which 

 the circular waves converge. This could be experimentally 

 realized with ripples by causing a circular arc and a style to 

 be agitated by the same prong of a fork, when the effects 

 would be analogous to M. Meslin's experiment in Optics. 



Fig. 3. Frequency 256. 



This photograph is very similar to fig. 1 ; but in addition 

 to showing interference phenomena like those of Fresnel and 

 Young, it also illustrates interference effects in which the 

 direction of propagation of light is parallel to the line joining 

 the point-sources. Thus in the photograph, if we consider 

 the disturbance anywhere on a right line drawn perpendicular 

 to a line joining the two point-sources produced, we see that 

 the places of no disturbance are symmetrical about the line 

 joining the sources. They are points on the system of 

 hyperbolas already mentioned. 



In M. Meslin's method of producing interference-fringes 

 the screen is placed between the two point-centres, one a 

 source and the other a sink. The bands are circular, and are 

 sections of ellipsoids of revolution, and not of hyperboloids, 

 such as the fringes in the photograph would become if the 

 whole picture were rotated about the line joining the point- 



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