186 Apparatus to illustrate the Interference of two Plane Waves. 



the board D as it moves along carries with it the sHding 

 board F with the attached pencil ; and as this board F is car- 

 ried along it rises and falls to the elevations and depressions 

 of the wave W^ But, as has been said, it also rises and falls 

 to the wave W : the motion of the pencil is therefore the alge- 

 braic sum of the forms of waves W and W^ In the figure 

 the waves W and W^ are of the same length and amplitude 

 and correspond in phase, so that the tracing W^^ is a wave of 

 the same length but twice the amplitude of the waves below. 

 By moving the wave W^ to the right, its phase may be 

 made |, i, &c. of an undulation different, and a fresh tracing 

 obtained. By removing the wave W^ and inserting succes- 

 sively others of different lengths and amplitudes, various com- 

 bined waves are traced out on moving the board D along. A 

 few tracings obtained bv the apparatus are represented in 

 tiff. 2. 



The wave C results in each case from the superposition of the waves A 

 and B immediately below C. 



In order that the frame carrying the wave W^ may rise and 

 fall freely, the brackets K, K are linked together at the back 

 of the board by a parallel motion, used in the Cowper printing- 



