ELECTRIC-SPARK PHOTOGRAPHS. 177 



report ou the water waves and their properties, I found that he had 

 made experiments and had j^iven a diagram showing- what happens 

 when a solitary wave meets a vertical wall. The wave, as wonld be 

 expected, is, under ordinary conditions, reflected perfectly, making an 

 angle of reflection equal to the angle of incidence, and the reflected and 

 incident waves are alike in all respects. This continues to be the case 

 as the angle gets more and more nearly equal to the right angle, *. e., 

 until the wave front, nearly perpendicular to the wall, runs along nearly 

 parallel to it. It then at last ceases to be reflected at all. The part of 

 the wave Jiear the wall instead gathers strength, it gets higher, it there- 

 fore travels faster, and so causes the wave near the wall to run ahead 

 of its proper position, producing a bend in the wave front, and this 

 goes on until at last the wave near the wall becomes a breaker. 



In order to see if anything similar happens in the case of air waves, 

 I arranged the three reflecting surfaces of sheet copper seen in Plate 

 VII, and photographed a magazine-rifle bullet when it had got to 

 the position seen. Below the bullet two waves strike the reflector at a 

 low angle, and they are perfectly reflected, the dark and the light lines 

 changing places as they obviously ought to do. The left side of the V- 

 shaped reflector was met at a nearly grazing incidence; there there is 

 no reflection, but, as is clear on the photograph, the wave near the 

 reflector is of greater intensity, it has bent itself ahead of its proper 

 position as the water wave was found to do, but it can not form a 

 breaker, as there is no such thing in an air wave. The same photo- 

 graph shows two other phenomena which are of interest. The stern 

 wave has a piece cut out of it by the lower reflector, and bent up at 

 the same angle. Now if a wave was a mere advancing thing the end 

 of the bent-up piece would leave off snddenly, and the break in the 

 direct wave would do the same. But according to the view of wave 

 propagation put forward by Iluygens, the wave at any epoch is the 

 resultant of all the disturbances which may be considered to have 

 started from all points of the wave front at any preceding epoch. 

 The reflector, where it has cut this wave, may be considered as a series 

 of points of disturbance arranged continuously in a line, each how- 

 ever coming into operation just after the neighbor on one side and 

 just before the neighbor on the other. The reflected wave is the enve- 

 lope of a series of spheres beginning with a point at the place where 

 the wave and the reflector cut, growing up to a finite sphere about the 

 end of the reflector as a center; beyond this there are no more centers 

 of disturbance, the envelope of all the spheres projected upon the 

 plate, that is, the photograi)h of the reflected wave, is not therefore a 

 straight line leaving off abruptly, but it curls round, as is very clearly 

 shown, dying gradually away to nothing. The same is the case, but it 

 is less marked, at the end of the direct wave near the part that has 

 been cut out. 



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