164 
POPULAR SCIENCE REVIEW. 
the ingenious suggestion of Mr. Wheatstone, which consists in 
causing the image of the flame to travel over different parts of 
the retina, by looking at the reflection in a moving mirror. 
Examined in this way, a steady jet of gas gives an uninterrupted 
band of light, like a glowing stick whirled in the air, but the 
continuity is broken when the flame begins to roar, for then a 
number of images are seen with dark spaces in between.* 
In this case the flame is nearly blown out by the rush of gas, 
but rekindled before it is quite extinguished, and this alternate 
action, rapidly kept up, constitutes the flicker of a candle and 
the roar of a gas flame. When roaring, therefore, a flame is in 
a state of rapid vibration, and it is this vibration which pro- 
duces the succession of images seen in a moving mirror. 
Now, imagine a sound to be made in a room wherein a steady 
jet of gas is burning ; the flame, along with other things in the 
room, is thrown into a state of vibration corresponding with the 
sound. The flame, being most easily disturbed, will be the 
most agitated, but will not be perceptibly affected, unless it be 
near its sensitive point. Now, permit an increased flow of gas 
to issue from the burner, so that the flame shall be just at its 
sensitive point, that is, if the gas ripples a little faster through 
the orifice the flame will change its shape and be thrown into a 
state of vibration. Let a sound be again made — if the vibra- 
tions now excited in the flame happen to be synchronous with 
those which the flame only lacked, in order to make it roar, 
then, those vibrations being superinduced by an extraneous 
source, the flame will accompany its response to the sound by a 
change in its shape. Hence, a sensitive flame is the analogue 
of a resonant column of air; both are caused insensibly to 
vibrate at any note, but when the pitch of the note accords 
with the normal rate of vibration of the flame or the air, then 
the flame visibly, and the column of air audibly, responds with 
energy to that note. So that bringing the flame to the point at 
which it is sensitive to a particular note, is like adjusting the 
length of a column of air until it resounds to a certain tuning- 
fork. 
'fhat a sensitive flame, under the influence of sound, is in a 
state of rapid vibration can readily be proved. Thus, the flames 
shown at fig. 2 and fig. 3 on the plate, when regarded in a 
moving mirror, are seen to consist of a succession of flames, 
* By varying the motion of the mirror, this effect can he rendered very 
beautiful, and nothing is simpler to reproduce. A. bit of looking-glass 
twisted to and fro in the hand before a dickering candle, or a roaring gas- 
flame resolves these common objects into luminous chains and crowns of 
weird and matchless beauty. It is indeed surprising, how we neglect the 
beauty which is so lavishly scattered through the great treasure-house of 
nature. 
