THE WAVE THEORY OF LIGHT. 311 



your left to your right, as you look at it. Look 

 at the motion when made faster. We have now 

 the direction reversed. The propagation of the 

 wave is from right to left, again the propagation 

 of the wave is from left to right ; each particle 

 moves perpendicularly to the line of propagation. 



I have given you an illustration of the vibration 

 of sound waves, but I must tell you that the move- 

 ment illustrating the condensation and rarefaction 

 I represented in that moving diagram are necessarily 

 very much exaggerated, to let the motion be 

 perceptible, whereas the greatest condensation in 

 actual sound motion is not more than one or two 

 per cent, or a small fraction of a per cent. Except 

 that the amount of condensation was exaggerated 

 in the diagram for sound, you have in the chart 

 a correct representation of what actually takes 

 place in sounding the low note C. 



On the other hand, in the moving diagram re- 

 presenting light waves what had we ? We had a 

 great exaggeration of the inclination of the line 

 of particles. You must first imagine a line of 

 particles in a straight line, and then you must 



