INTERFERENCE OF LIGHT 323 



fig. 175 was however chiefly designed to illustrate inter- 

 ference, which it does in a manner superior to any other. 

 Instead of the one simple wave-glass just described, when 

 demonstrating interference the sliding part is divided into 

 three strips as shown in fig. 176, kept together edge to edge 

 in one plane by a light binding or frame at each end, which 

 is cemented to the two outer strips, but allows the middle one 

 to be moved endways. On one outside strip of blackened 

 glass is cut a wave, and on the other a wave of double the 

 length ; and the middle strip has a wave of each length, 

 as shown in the figure. It is convenient to colour over the 

 long waves red and the short ones blue, with transparent 

 colour. All the 

 waves are first 

 shown in similar 

 phases, pointing 

 out that two vi- 

 brations of the 

 short ones take 

 place in the same 



time as One Of the PIG. 176. Interference Slide 



long ones. Then 



by drawing along the centre strip half the length of the longer 

 wave, it will be seen how this measured retardation of one 

 pair of waves brings the long (red) waves into opposite phases, 

 whilst the short ones (blue) are still in the same phase. 



The superposition as well as destruction of vibration may 

 be shown by such a model as is used at the Koyal Institution. 

 A wave is cut in the top of a board standing on edge. Any 

 number of thin rods sliding perpendicularly in the same plane 

 in a light framework, with narrow spaces between, are cut 

 of such a length that, when standing on a flat surface, their 

 tops give a similar wave. When the frame of rods is placed 

 so that the longer rods stand on the hollows in the top of 

 the board, the profile at the top is horizontal, or the wave is 



