158 Prof. H. M'Leod and Lieut. G. S. Clarke on [Apr. 19, 



fork vibrates, and parallel to the plane of the mirror, the combination of 

 the two rectilinear motions will produce a sinuous line or wave-form. 



The dimensions of this wave will depend on the amplitude of the 

 vibrations of the fork and on the velocity of movement of the point of 

 light in relation to the period of the fork : when the rate of translation 

 of the point is great the wave-length will be great ; with a low velocity 

 the wave-length will be small. The greater the amplitude of the vibra- 

 tions of the fork, the greater will be the amplitude of the wave. 



Yfhen a series of equidistant points attached to a rotating disk or 

 cylinder is employed, the properties of the wave differ very much accord- 

 ing to the velocity of the mo^ng disk. If the points are placed in a 

 circle on a disk or cylinder rotating with such a velocity that the time 

 occupied by a point in passing over a distance equal to that between two 

 consecutive points is exactly equal to the period of one complete vibration 

 of the fork, a continuous stationary figure is perceived ; but if the point 

 passes over a distance slightly greater than the intervals, the figure will 

 show a slow progression in the direction of the moving circle ; and when 

 the space described by the point is slightly less than the distance between 

 the points, the motion of the figure will be in the opposite direction. If 

 the velocity of the circle is one half of that necessary to produce the sta- 

 tionary wave another stationary figure results, but with half the wave- 

 length of the previous one, the luminous point passing over a distance 

 equal to the interval between two points during two vibrations of the 

 fork ; the brightness of the wave is also less. Generally, if the time oc- 

 cupied by the point in moving through a distance equal to that between 

 two consecutive points is an exact multiple of the period of the fork, 

 stationary waves will be formed ; waves thus produced have been observed 

 with velocities §, J, J, 4-, and -J- of that necessary to generate the wave-form 

 first described. 



When the velocity of the moving points is so much increased that the 

 eye is una,ble to follow the movement of the figure, a complex form is pro- 

 duced by the overlapping of several waves ; and at some velocities these 

 figures may appear stationary, but their complexity usually makes it 

 difficult to establish their exact form. Another simple and easily recog- 

 nized figure, however, is observed when the velocity of the moving points 

 is once and a half as great as that necessary to produce the stationary 

 single wave. This figure is caused by the overlapping of three waves 

 so placed that the crest of one is over the crossing of the other two. 

 This figure, like the previous one, shows a direct or inverse motion when 

 the velocity of the points is not exactly that which would produce the 

 stationary figure. When the velocity of the points is such that the dis- 

 tance traversed by one point during a complete vib:ation of the fork is 

 equal to double the interval between two consecutive points, a figure is 

 formed by the overlapping of two waves with their crests and hollows 

 opposed and crossing at the line that would be described by the points if 



