THE PHOTOGRAPHY OF SOUND WAVP^S AND THE DEMON- 

 STRATION OF THE EVOLUTIONS OF REFLECTED WAVE 

 FRONTS WITH THE CINEMATOGRAPH. ^ 



Bv R. W. Wood. 



INTRODUCTION. 



In a paper published in the Philosophical Magazine for August, 1899, 

 I gave an account of some experiments on the photography of sound 

 waves, and their application in the teaching of optical phenomena. 

 Since writing this paper I have extended the work somewhat, and at 

 a meeting of the Royal Society, on Februaiy 15, 1900. gave an account 

 of this work and deiuonstrated certain features of wave motion with 

 the cinematograph. 



In the present article I propose to give a somewhat more extended 

 account of the work, paying especial attention to the analogies between 

 the sound waves and waves of light. 



In teaching the subjects of optics we are obliged to resoi't to dia- 

 grams when dealing with the wave front, and in spite of all that we 

 can do the student is apt to form the opinion that the rays are the 

 actual entities, and that wave fronts are after all merely conceptions. 



The set of photographs illustrating this article Avill, I think, be of 

 no small use to teachers in ridding the minds of students of the 

 obnoxious rays, and impressing the fact that all of the common phe- 

 nomena of reflection, refraction, and diflraction are due simply to 

 changes wrought on the wave front. 



Sound waves in air were first observed and studied by Toepler. by 

 means of an exceedingh' sensitive optical contrivance for rendering 

 visible minute changes in the optical density of substances. A very 

 full description of the device will be found in Toepler's article (Wied. 

 Annalen, cxxxi), while a brief account of it will be given presently. 



The waves in question are the single pulses of condensed air given 

 out by electric sparks. A train of waves would complicate matters 

 too much, and for illustrating the optical phenomena which we are to 

 take up would be useless. 



The snap of the spark gives us just what we require, namely, a sin- 

 gle wave front, in which the condensation is considerable. 



1 Reprinted from Nature, No. 1606, vol. 62, August 9, 1900. 



359 



