246 ANNUAL OF SCIENTIFIC DISCOVERY. 



separated into a series of distinct images ; the distance between the images 

 depended upon the velocity with which the head was moved. This experi- 

 ment is suited to a darkened lecture-room. It was still easier to obtain the 

 separation of the images in this way, when a tube six feet nine inches in 

 length and a larger flame were made use of. 



As suggested to me by a lady to whom I had the pleasure of 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 thirty-three 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 six or eight 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 continu- 

 ous luminous track. Holding the mirror still, the six foot nine inch tube 

 was placed over the flame ; the latter changed its shape the moment it com- 

 menced to sound, remaining, however, well defined upon the screen. On 

 now moving the mirror, a totally different effect was produced ; instead of 

 a continuous track of light, a scries 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 three faces received in succession the beam of light sent 

 from the flame through the lens in front of it, and through the images upon 

 the screen. On commencing its motion, the images were but slightly sepa- 

 rated, but became more and more so as the motion approached its maximum. 

 This once past, the images drew closer 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 lampblack, so as to cut off the direct light of the 

 jet from the screen. 



But what is the state of the flame in the interval between two images 1 

 The flame of common gas or of defiant gas, owes its lurninousness to the 

 solid particles of carbon discharged into it. If we blow against a luminous 

 gas flame, a sound is heard, a small explosion in fact, and by such a puff 



