2B8 Prof. E. W. Wood. Photography of Sound-ivaves, and the 



arc FG (the path of the centre of the generating circle), and describing 

 circles of diameter BE around various points on it. A line joining 

 the point of intersection of one of these circles with the epicycloid, 

 and the point of tangency with the mirror, will, when produced, give 



A reflected ray ; for example, 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 Kne, which represents the wave-front. After a time it 

 becomes necessary to consider double reflections, and to do this we are 

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

 and measure around a corner each time. 



About a hundred pictures are prepared for each series, and the 

 pictures then photographed separately on the film, which, when run 



