188 Lord Bayleigh [Jan. 20, 



sharj^er still by diminishing the source of light. Sound-shadows, as 

 I have said, are not often sharp ; but I believe that they are sharper 

 than is usually supposed, the reason being that when we pass into a 

 sound-shadow — when, for example, we pass into the shade of a large 

 obstacle, such as a building — it requires some little time to effect the 

 transition, and the consequence is that we cannot make a very ready 

 comparison between the intensity of the sound before we enter and its 

 diminution afterwards. When the oomj)arison is made under more 

 favourable conditions, the result is often better than would have been 

 expected. It is, of course, impossible to perform experiments with 

 such obstacles before an audience, and the shadows which I propose 

 to show you to-night are on a much smaller scale. I shall take 

 advantage of the sensitiveness of a flame such as Professor Tyndall 

 has often used here — a flame sensitive to the waves produced by notes 

 so exceedingly high as to be inaudible to the human ear. In fact, 

 all the sounds with which I shall deal to-night will be inaudible to 

 the audience. I hope that no quibbler will object that they are there- 

 fore not sounds : they are in every respect analogous to the vibrations 

 which produce the ordinary sensations of hearing. 



I will now start the sensitive flame. We must adjust it to a 

 reasonable degree of sensitiveness. I need scarcely explain the 

 mechanism of these flames, which you know are fed from a special 

 gasholder supplying gas at a high pressure. When the pressure is 

 too high, the flame flares on its own account (as this one is doing 

 now), independently of external sound. When the pressure is some- 

 what diminished, but not too much so — when the flame " stands on 

 the brink of the precipice," were, I think, Tyndall's words — the 

 sound pushes it over, and causes it to flare ; whereas, in the absence 

 of such sound, it would remain erect and unaffected. Now, I believe, 

 the flame is flaring under the action of a very high note that I am 

 producing here. That can be tested in a moment by stopping the 

 sound, and seeing whether the flame recovers or not. It recovers now. 

 What I want to show you, however, is that the sound shadows may 

 be very sharp. I will put my hand between the flame and the source 

 of sound, and you will see the difterence. The flame is at present 

 flaring ; if I put my hand here, the flame recovers. When the adjust- 

 ment is correct, my hand is a sufficient obstacle to throw a most 

 conspicuous shadow. The flame is now in the shadow of my hand, 

 and it recovers its steadiness : I move my hand up, the sound comes 

 to the flame again, and it flares. When the conditions are at their 

 best, a very small obstacle is sufficient to make the entire difference, 

 and a sound shadow may be thrown across several feet from an 

 obstacle as small as the hand. The reason of the divergence from 

 ordinary experience here met with is, that while the hand is a fairly 

 large obstacle in comparison with the wave-length of the sound I am 

 here using, it would not be a sufficiently large obstacle in comparison 

 with the wave-lengths with which we have to do in ordinary life and 

 in music. 



