the Photography of Ripples. 343 



makes a small angle a with the horizontal, then the deviation 



produced by refraction is, in the case of water, - while for 



o 



reflexion it is 2«. In consequence, both the spark and the 



aperture of the stop must be made as small as possible. The 



stop used was F/61. 



Description of Photographs. 



The figures 1, 2, 6, and 7 are on the same scale as the 

 original negatives, and are approximately | natural size. 

 The others are ^ natural size. The frequency is about 80 in 

 all cases. 



Fig. 1. This illustrates the refraction of a beam of parallel 

 light at a plane surface. The longer wave-length corresponds 

 to the deep region, w r hile the shallow portion is enclosed b} r 

 the two parallel straight lines, which are the photographs of 

 the edges of the piece of glass which in this case rests on the 

 floor of the trough. The waves are started in the deep region, 

 on meeting the boundary-line they suffer both refraction and 

 reflexion. The evidence of reflexion can just be made out on 

 the photograph. 



Fig. 2. This illustrates the passage of light through a 

 prism. A triangle of glass forming the floor of the shallow 

 portion is seen in the photograph. The waves bend on entrance 

 to and emergence from the shallow triangular patch, and a 

 marked deviation is produced. 



Fig. 3. The rectangle is the photograph of a portion of a 

 short circular cylinder of glass placed in the trough, so that 

 its convex surface is just submerged. The axis of the 

 cylinder is parallel to the shorter side of its rectangular plan. 

 The circular waves which originate in deep water are retarded 

 by passing over the shallow portion. The interference-fringes 

 caused by the refraction are somewhat analogous to those 

 formed by a Fresnel biprism. 



Fig. 4. The depth here increases gradually as we pass 

 away from the source in one direction. The source is placed 

 immediately over the edge of a glass plate. This plate is sub- 

 merged, and has one straight edge almost in the surface of the 

 liquid. The waves on the deep side are thus semicircular, 

 while on the water covering the sloping plate they are dis- 

 torted by the retardation in the shallow parts. 



Fig. 5. In this the point-source is placed over the sloping 

 floor some distance from the shallow line. 



Fig. G. Circular waves from a point-source travel into a 



