illustrate the Interference of two Plane Waves. 185 



and quickly placed in position by letting down the flaps F, F^ 

 D is a stout mahogany board, with a wedge-shaped groove in 



Fi^. 1. 



Moving the boards D, E, F from right to left by means of the handle 

 H traces the wave W", a combination of the waves W and W. 



it sufficiently wide to allow the board E to slide freely up and 

 down. A handle, H, is secured to the board D to enable the 

 operator to move D from one end of A to the other. The 

 board E has a pulley fastened to its lower part ; and this pul- 

 ley runs on the tin wave W, so that if the board D be moved 

 horizontally, the sliding board E will rise and fall as the pul- 

 ley passes over W ; and of course, were there a pencil attached 

 directly to E, the pencil would trace a curve the same, or very 

 nearly the same, as that of the wave. The sliding board E 

 has also a wedge-shaped groove cut in it to allow the board F 

 to slide vertically ; and to F is also attached a pulley, which 

 runs on the wave W^ With the upper board F, as with the 

 lower, if the pulley be made to traverse the wave W^, a pencil 

 fastened to F would trace a wave corresponding to W^. The 

 pencil (or, more properly, pen) used in the apparatus consists 

 of a thistle-funnel, with the tube drawn out so as to have an 

 opening of only a fraction of a millimetre. The tube is bent 

 as shown in the figure at G ; and the drawn-out part presses 

 gently against the cardboard C. To charge the pen, some 

 violet ink is poured into the thistle-funnel, which is then closed 

 with a cork. 



The action of the apparatus will easily be understood. As 

 the board D moves along, the board E rises and falls ; and this 

 by means of a roller causes the wave W^ to rise and fall to 

 each elevation and depression of the wave W below. Now 



