NATURAL PHILOSOPHY. 247 



the lurainousness may be caused to disappear. During a windy night the 

 exposed gas-jets in the shops are often deprived of their light and burn blue. 

 In like manner the common blowpipe jet deprives burning coal gas of its 

 brilliant light. I hence concluded that the explosions, the repetition of 

 which produces the musical sound in the case before us, rendered, at the 

 moment they occurred-, the combustion so perfect as to extinguish the solid 

 carbon particles ; but I imagined that the images on the screen would, on 

 closer examination, be found united by spaces of blue, which, owing to their 

 dimness, were not seen by the method of projection. This, in many instances, 

 was found to be the case. 



I was not, however, prepared for the following result : A flame of ole- 

 fiant gas, rendered almost as small as it could be, was procured. The three 

 feet two inch tube was placed over it ; the flame on singing became elong- 

 ated and lost some of its light, still it was bright at its top ; looked at in 

 the moving mirror, a beaded line of great beauty was observed ; in front of 

 each bead was a little luminous star ; after it, and continuous with it, a spot 

 of rich blue light, which terminated and left, as far as I could judge, a per- 

 fectly dark space between it and the next following luminous star. I shall 

 examine this further when time permits me ; but, as far as I can 'at present 

 judge, the flame was actually extinguished and re-lighted in accordance with 

 the sonorous pulsations. 



When a silent flame, capable, however, of being excited by the voice in 

 the manner already described, is placed within a tube, and the continuous 

 line of light produced by it in the moving mirror is observed, I know no ex- 

 periment more pretty than the resolution of this line into a string of richly 

 luminous pearls at the instant when the voice is pitched to the proper note. 

 This may be done at a considerable distance from the jet, and with the back 

 turned towards it. 



The change produced in the line of beads when a tuning-fork, capable of 

 giving beats with the flame, is brought over the tube, or over a resonant jar 

 near it, is also extremely interesting to observe. I will not at present enter 

 into a more minute description of these results. Sufficient, I trust, has been 

 said to induce experimenters to produce the effects for themselves ; the sight 

 of them will give more pleasure than any description of mine could possi- 

 bly do. 



ON THE OPTICAL STUDY OF VIBRATIONS. 



A memoir has been recently presented to the French Academy, by M. 

 Lissaijous " on the Optical Study of Vibrations." The author proposes, 

 in judging of the number of vibrations which occasion particular sounds, to 

 reject all appeals to the ear, and to judge of the vibratory motions by the 

 eye. He places on the exterior face of a tuning-fork a small mirror, and 

 directs a sun-beam upon it, which is reflected upon a paper screen placed in 

 the angle of its reflection ; the beam is concentrated by a convergent lens. 

 As long as the tuning-fork is silent the image is motionless, but if it is vi- 

 brated the reflected ray vibrates in the same plane, and its extremity, rapidly 

 oscillating on the screen, traces a lengthened image, whose superficies is in 



