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THREE-DIMENSIONAL MOTION PICTURES ^9 The simplest example of wave motion is that of the vibra tions Imparted to a taut string. If a shock is produced at one end of the string the whole string is set into a vibratory motion which from its start, at the point where the disturbance is first created, is propagated to the other end and is continuous, until the influence of the distrubance ceases to have effect and the string is again in a state of rest. It is seen that the string Is not displaced and that the waves are formed by a change of position of each of the points of the string. Their propagation is very similar to the propagation of the waves created on the surface of stagnant water by any dis turbance. The appellation waves is applied to all disturbances in which a particular state of a medium is brought on without change in the medium and it originates from the familiar effect of waves produced on the surface of water. Suppose that a cork is lying on the surface of a calm pool and the surface of the water is touched at any other point by means of a stick or by throwing a pebble into the pool. A cir cular system of waves will be formed having as center the point of disturbance. These waves will have a transversal motion and gradually enlarge their circle, the extreme boundary of which will at any definite time mark the wave front of the dis turbance. When these waves reach the cork the latter will be seen to follow the transversal motion but to remain in the same spot. Light waves are propagated in a very similar manner. The length of the light wave has a marked influence on the quality of the light, as waves of different length have a dif ferent repercussion upon the eye and establish the different sensations called colors. As the light waves are propagated with a finite velocity it is easily understood that other quantitative entities can be ex pressed and this in respect as to time and space. The first of these entities is the number of waves that are created in a certain fixed time. This has been called the jre-