478 Prof. Tyndall on the Sounds produced by 



showing these experiments, the same result is obtained when an 

 opera glass is moved to and fro before the eye. 



But the most convenient mode of observing the flame is with a 

 mirror ; and it can be seen either directly in the mirror, or by 

 projection upon a screen. 



A lens of 33 centimeters focus was placed in front of a flame 

 of common gas, upwards of an inch long, and a paper screen was 

 hung at about 6 or 8 feet distance behind the flame. In front of 

 the lens, a small looking-glass was held which received the light 

 that had passed through the lens, and reflected it back upon the 

 screen placed behind the latter. By adjusting the position of 

 the lens, a well-defined inverted image of the flame was obtained 

 upon the screen. On moving the mirror the image was displaced, 

 and owing to the retention of the impression by the retina, 

 when the movement was sufficiently speedy, the image described 

 a continuous luminous track. Holding the mirror still, the 

 6-foot 9-inch tube was placed over the flame : the latter changed 

 its shape the moment it commenced to sound, remaining how- 

 ever well defined upon the screen. On now moving the mirror, 

 a totally different effect was produced : instead of a continuous 

 track of light, a series of distinct images of the sounding flame 

 was observed. The distance of these images apart varied with 

 the motion of the mirror ; and, of course, could be made, by 

 suitably turning the reflector, to form a ring of images. The 

 experiment is beautiful, and in a dark room may be made visible 

 to a large audience. 



The experiment was also varied in the following manner: — A 

 triangular prism of wood had its sides coated with rectangular 

 pieces of looking-glass : it was suspended by a thread with its 

 axis vertical ; torsion was imparted to the thread, and the prism 

 acted upon by this torsion caused to rotate. It was so placed 

 that its thi'ce faces received in succession the beam of light sent 

 from the flame through the lens in front of it, and threw the 

 images upon the screen. On commencing its motion the 

 images were but slightly separated, but became more and more 

 so as the motion approached its maximum. This once past, the 

 images drew clogier together again, until they ended in a kind 

 of luminous ripple. Allowing the acquired torsion to act, the 

 same series of effects could be produced, the motion being in an 

 opposite direction. In these experiments, that half of the tube 

 which was turned towards the screen was coated with lamp- 

 black, so as to cut off the direct light of the jet from the screen*. 



* Since these experiments were made, Mr. Wheatstone has di-awn my 

 attention to the following passage, which proves that he was the first to 

 make use of the rotating mirror in examining a singing flame : — " A flame 

 of hydrogen gas burning in the open air presents a continuous circle in the 

 muTor ; but while producing a sound within a glass tube, regular inter- 

 missions of intensity are observed, which present a chain-like appearance, 



