1857.] Acoustic Experiments, 448 



tions of both the forks. This is the principle of the entire series 

 of experiments now to be referred to. 



When a single fork vibrates, the image which it casts upon the 

 screen is elongated in a direction parallel to the prong of the fork. 

 In order to have the vibrations rectangular, one fork stood upright, 

 the other was fixed horizontally, in a vertical stand, in the following 

 experiments. 



5. Two forks, in perfect unison with each other, were placed 

 in the positions described, and caused to vibrate simultaneously. 

 If both forks passed their position of equilibrium at the same 

 instant, that is, if there was no difference of phase, the figure 

 described was a straight line. When the difference of phase 

 amounted to one-fourth, the figure was a circle : between these 

 it was an ellipse. The perfect unison of the two forks was proved 

 by the immobility of the figure upon the screen. On loading one 

 of them with a little weight, the figure no longer remained fixed 

 but passed from the straight line through the ellipse to a circle, 

 thence back through the ellipse to the straight line. So slight is 

 the departure from unison which may be thus rendered visible, that 

 M. Lissajous states that it would be possible to make evident to a 

 deaf person a discrepancy of one vibration in thirty thousand. 



6. Two forks, one of which gave the octave of the other, were 

 next made use of. When there was no difference of phase, the 

 figure described upon the screen resembled an 8. If the unison 

 was perfect, the figure, as in the former case, was fixed ; but when 

 the unison was disturbed, the figure passed through the changes cor- 

 responding to all possible differences of phase. The loops of the 

 8 became distorted, formed by superposition a single parabola, 

 opened out again, became again symmetrical, and so on. 



7. The fifth of the octave, the major third, and other combina- 

 tions succeeded, the figures becoming more and more complex as 

 the departure from simple relations between the vibrations in- 

 creased. 



8. Finally, two forks which, when sounded together, gave 

 audible beats, were placed both upright upon the table. The 

 beam reflected from the mirror of one was received upon that of 

 the other, and reflected upon the screen. When both forks were 

 sounded, they sometimes conspired to elongate the image ; some- 

 times they opposed each other, and thus a series of elongations and 

 shortenings addressed the eye at exactly the same intervals in which 

 the beats addressed the ear. 



At the conclusion of this beautiful series of experiments, which, 

 thanks to the skill of those who performed them, were all successful, 

 on the motion of Mr, Faraday, the thanks of the meeting were 

 unanimously voted to M.M. Lissajous and Duboscq, and commu- 

 nicated to those gentlemen by his Grace the President. 



[J.T.] 



