March 9, 1893] 



NA TURE 



441 



of air waves, I arranged the three reflecting surfaces of 

 sheet copper seen in Fig. ii,and photographed a maga- 

 zine 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 cannot form a breaker, as there is no such thing in 

 an air wave. The same photograph 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 



flector cut, growing up to a finite sphere about the end of 

 the reflector as a centre ; beyond this there are no more 

 centres of disturbance, the envelope of all the spheres 

 projected upon the plate, that is, the photograph 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. 



The other point to which I would refer is the dark line 

 between the nose of the bullet and the wire placed to 

 receive it. This is the feeble spark due to the discharge 

 of the small condenser which clearly must have been on 

 the point of going off of its own accord. The feeble 

 spark precedes or is to all intents and purposes simul- 

 taneous with, it cannot follow — the main spark which 



thing the end of the bent-up piece would leave off 

 suddenly, and the break in the direct wave would do the 

 same. But according tb the view of wave propagation 

 put forward by Huygens, the wave at any epoque is the 

 resultant of all the disturbances which may be considered 

 to have started from all points of the wave front at any 

 preceding epoque. The reflector, where it has cut this 

 wave, may be considered as a series of points of disturb- 

 ance arranged continuously in a line, each, however, 

 coming into operation just after the neighbour on one side 

 and just before the neighbour on the other. The reflected 

 wave is the envelope of a series of spheres beginning 

 with a point at the place where the wave and the re- 

 NO. I 2 19, VOL. 47] 



makes the photograph. The feeble spark heated the air, 

 and the light from the main spark coming through this 

 line of heated air was dispersed, leaving a clear black 

 shadow on the plate. One spark casts a shadow of the 

 other. Now it is evident that if the spark at the nose of 

 the bullet had followed instead of having preceded the 

 main spark by even so much as a three-hundred-millionth 

 of a second, the time that light took to travel from one to 

 the other, it would not have been able to cast a shadow. 

 We have the means of telling, therefore, which of two 

 sparks actually took place first, or perhaps the order of 

 several, even though the difference of time is so minute. 

 Perhaps this method might be of some use in researches- 



