154 Prof. R. W. Wood on Photography of Sound- Waves 



surface, arid normals can be erected by drawing the arc FG 

 (the path of the centre of the generating circle), and de- 

 scribing circles of diameter BE around various points on it. 

 A line joining the point of intersection of one of these circles 



A 













/ y 













*r / 1 

 / / 



_____ — ■*"""""" • / 







^Ik^T 





x" 













B 







\^ 



















O 





with the epicycloid, and the point of tangency with the 

 mirror, will, when produced, give a reflected ray ; for ex- 

 ample, JK produced for circle described around H. This 

 construction once prepared, the series of wave-front pictures 

 can be very quickly made. Three or four sheets of paper 

 are laid under the construction and holes are punched through 

 the pile by means of a pin, at equal distances along each ray 

 (measured from the orthogonal surface). 



The centre of the mirror and the point where its axis 

 meets the surface are also indicated in the same manner. 

 The sheets are now separated, and corresponding pin-holes 

 are united on each sheet by a broad black line, which repre- 

 sents the wave-front. After a time it becomes necessary to 

 consider double reflexions, and to do this we are compelled 

 to construct twice-reflected rays (indicated by dotted lines), 

 and measure around a corner each time. 



