442 Professor Tyndall, on M. Lissajous* [June 5, 



his experiments before the Societe d'Encouragement, and more 

 recently before the' Emperor of the French. When he became 

 acquainted with the speaker's intention to introduce these ex- 

 periments at the Royal Institution, he in the most obliging manner 

 offered to come to London and make them himself. This offer was 

 accepted, and the speaker also congratulated the audience on the 

 presence of M. Duboscq, who took charge of his own electric 

 lamp ; this being the source of light made use of on the occasion. 

 The experiments proceeded in the following order : — 



1. A sheaf of light was thrown from the lamp upon a mirror 

 held in the speaker's hand : on moving the mirror with sufficient 

 speed the beam described a luminous ring upon the ceiling. The 

 persistance of impressions upon the retina was thus illustrated. 



2. A tuning fork had a pointed bit of copper foil attached to 

 one of its prongs : the fork being caused to vibrate by a violin bow 

 the metallic point moved to and fro, and being caused to press 

 gently upon a surface of glass coated with lamp black, the fork 

 being held still, a fine line of a length equal to the amplitude of the 

 vibrations was described upon the glass ; but when at the same 

 time the whole fork was drawn backwards with sufficient speed, a 

 sinuous line was described upon the glass. The experiment was 

 made by placing the coated glass before the lamp ; having a lens 

 in front of it, and bringing the surface of the glass to a focus on a 

 distant screen. On drawing the fork over the surface in the man- 

 ner described, the figure started forth with great beauty and pre- 

 cision. By causing a number of forks to pass at the same time 

 over the coated glass, the relations of their vibrations were deter- 

 mined by merely counting the sinuosities. The octave, for example, 

 had double the number of its fundamental note. 



3. This was the first of the series of M. Lissajous' experiments. 

 A tuning fork, with a metallic mirror attached to one of its prongs, 

 was placed in front of the lamp ; an intense beam of light was 

 thrown on the mirror, and reflected back by the latter. This 

 reflected beam was received on a small looking-glass, held in the 

 hand of the experimenter, from which it was reflected back upon 

 the screen. A lens being placed between the lamp and tuning fork, 

 a sharply defined image of the orifice from which the light issued 

 was obtained. When a violin bow was drawn across the fork, 

 this image elongated itself to a line. By turning the mirror in the 

 hand, the image upon the screen was resolved into a bright sinuous 

 track, many feet in length. 



4. A tuning fork was placed before the lamp, as in the last ex- 

 periment. But instead of receiving the beam reflected from the 

 mirror of the fork upon a looking-glass, it was received upon the 

 mirror of a second fork, and reflected by the latter upon the screen. 

 When one fork was excited by a bow, a straight line described 

 itself upon the screen, when the other fork was subsequently excited, 

 the figure described was that due to the combination of the vibra- 



