7 2 THE ART OF PROJECTING. 



being equal to the sum of the two amplitudes. If their 

 phases are opposite, so that one reaches its upper limit 

 at the same instant that the other reaches its lower 

 limit, then the spot of light will not be drawn out into a 

 line at all, but will remain quiescent. These various 

 relative vibrations can only be obtained by trial, but 

 usually in four or five strokes one will develop such a 

 phase as he requires. 



IV. Resultants. Keeping the two forks in unison, 

 turn the second fork so that it vibrates horizontally. 

 Adjust the light so that it falls upon the second mirror 

 as before, and thence to the middle of the screen. 

 Now, if both forks be struck, the resulting figure may 

 be a straight line, an ellipse, or a circle depending 



F! ff . 50. 



upon the phase of the first fork when the second one 

 begins to vibrate. Fig. 50 represents these unison 

 forms. By moving one of the slides so that the fork is 

 not quite in tune with the other, the figure will move 

 through each of these phases alternately. When the 



Fig. Bl. 



circle is obtained upon the screen, swing the second 

 fork through a small arc, and the circle will be drawn 

 out into a luminous scroll, (Fig. 51). If the forks are 



