June 28, 1888] 



NATURE 



209 



The question of whether the flame is excited at the nodes or 

 at the loops — whether at the places where the pressure varies 

 most, or at those where there is no variation of pressure, but 

 considerable motion of air — is one of considerable interest from 

 the point of view of the theory of these flames. The experiment 

 Could be made well enough with such a source of sound as I am 

 now using ; but it is made rather better by using sounds of a 

 lower pitch, and therefore of greater wave-length, the discrimina- 

 tion being then more easy. Here is a table of the distances 

 which the screen must be from the flame in order to give the 

 maximum and the minimum effect, the minimum being practically 

 nothing at all. 



Table of Maxima and Minima. 



Max. 

 PI 



4'5 



7'5 



IO-3 



130 



I5-9 



Min. 



3'o 

 5'9 

 8-9 

 117 

 H7 



The distance between successive maxima or successive minima 

 is very nearly 3 cm., and this is accordingly half the length of the 

 wave. 



But there is a further question behind. Is it at the loops or 

 is it at the nodes that the flame is most excited ? The table shows 

 what the answer must be, because the nodes occur at distances 

 from the screen which are even multiples, and the loops at 

 distances which are odd multiples ; and the numbers in the 

 table can be explained in only one way — that the flame is excited 

 at the loops corresponding to the odd multiples, and remains 

 quiescent at the nodes corresponding to the even multiples. 

 This result is especially remarkable, because the ear, when 

 substituted for the flame, behaves in the exactly opposite manner, 

 being excited at the nodes and not at the loops. The experi- 

 ment may be tried with the aid of a tube, one end of which is 

 placed in the ear, while the other is held close to the burner. It 

 is then found that the ear is excited the most when the flame is 

 excited least, and vice versa. The result of the experiment 

 shows, moreover, that the manner in which the flajne is dis- 

 integrated under the action of sound is not, as might be expected, 

 symmetrical in regard to the axis of the flame. If it were 

 symmetrical, it would be most affected by the symmetrical 

 cause — namely, the variation of pressure. The fact being that it 

 is most excited at the loop, where there is the greatest vibratory 

 velocity, shows that the method of disintegration is unsym- 

 metrical, the velocity being a directed quantity. In that respect 

 the theory of these flames is different from the theory of the 

 water-jets investigated by Savart, which resolve themselves into 

 detached drops under the influence of sonorous vibration. The 

 analogy fails at this point, and it has been pressed too far by 

 some experimenters on the subject. Another simple proof of 

 the correctness of the result of our experiment is that it makes 

 all the difference which way the burner is turned in respect of 

 the direction in which the sound-waves are impinging upon it. 

 If the phenomenon were symmetrical, it would make no 

 difference if the flame were turned round upon its vertical axis. 

 But we find that it does make a difference. This is the way in 

 which I was using the flame, and you see that it is flaring 

 strongly. If I now turn the burner round through a right angle, 

 the flame stops flaring. I have done nothing more than turn the 

 burner round and the flame with it, showing that the sound- 

 waves may impinge in one direction with great effect, and in 

 another direction with no effect. The sensitiveness occurs again 

 when the burner is turned through another right angle ; after 

 three right angles there is another place of no effect ; and after 

 a complete revolution of the flame the original sensitiveness 

 recurs. So that if the flame were stationary, and the sound- 

 waves, came, say, from the north or south, the phenomena 

 would be exhibited ; but if they came from the east or west, the 

 flame would make no response. 



This is of convenience in experimenting, because, by turning 

 the burner round, I make the flame almost insensitive to a 

 sound, and I am now free to show the effect of any sound that 

 may be brought to it in the perpendicular direction. I am going 



to use a very small reflector — a small piece of looking-glass. 

 Wood would do as well ; but looking-glass facilitates the adjust- 

 ment, because my assistant, by seeing the reflection, will be 

 able to tell me when I am holding it in the best position. Now, 

 the sound is being reflected from the bit of glass, and is causing 

 the flame to flare, though the same sound, travelling a shorter 

 distance and impinging in another direction, is incompetent to 

 produce the result (Fig. 1). 



I am now going to move the reflector to and fro along the 

 line perpendicular to that joining the source and the burner, all 

 the while maintaining the adjustment, so that from the position 

 of the source of sound the image of the flame is seen in the 

 centre of the mirror. Seen from the source, it is still as central 

 as before, but it has lost its effect, and as I move it to and fro I 

 produce cycles of effect and no effect. What is the cause of 

 this ? The question depends upon something different from what 

 I have been speaking of hitherto ; and the explanation is, that 

 we are here dealing with a diffraction phenomenon. The mirror 

 is a small one, and the sound-waves which it reflects are not big 

 enough to act in the normal manner. We are really dealing 

 with the same sort of phenomena as arise in optics when we use 

 small pin-holes for the entrance of our light. It is not very easy 

 to make the experiment in the present form quite simple, 

 because the mirror would have to be withdrawn, all the while 

 maintaining a somewhat complicated adjustment. In order to 

 raise the question of diffraction in its simplest shape, we must 

 have a direct course for the sound between its origin and the 

 place of observation, and interpose in the path a screen perforated 

 with such holes as we desire to try. 



The screen I propose to use is of glass. It is a practically 

 perfect obstacle for such sounds as we are dealing with ; but it 

 is perforated here with a hole (20 cm. diameter), rendered more 

 evident to those at a distance by means of a circle of paper 

 pasted round it. The edge of the hole corresponds to the inner 

 circumference of the paper. We shall thus be able to try the 

 effect of different-sized apertures, all the other circumstances re- 

 maining unchanged. The experiment is rather a difficult one 

 before an audience, because everything turns on getting the 

 exact adjustment of distances relatively to the wave-length. At 

 present the sound is passing through this comparatively large 

 hole in the glass screen, and is producing, as you see, scarcely 

 any effect upon the flame situated opposite to its centre. But if 

 (Fig. 2) I diminish the size of the hole by holding this circle of 

 zinc (perforated with a hole 14 cm. in diameter) in front of it, 

 it is seen that, although the hole is smaller, we get a far greater 

 effect. That is a fundamental phenomenon in diffraction. Now 

 I reopen the larger hole, and the flame becomes quiet. So that 

 it is evident that in this case the sound produces a greater effect 

 in passing through a small hole than in passing through a larger 

 one. The experiment may be made in another way, by ob- 

 structing the central in place of the marginal part of the aper- 

 ture in the glass. When I hold this unperforated disk of zinc 

 (14 cm. in diameter) centrically in front, we get a greater effect 

 than when the sound is allowed to pass through both parts 

 of the aperture. The flame is now flaring vigorously under 

 the action of the sonorous waves passing the marginal part 

 of the aperture, whereas it will scarcely flare at all under the 

 action of waves passing through both the marginal and the 

 central hole. 



This is a point which I should like to dwell upon a little, 

 for it lies at the root of the whole matter. The principle upon 

 which it depends is one that was first formulated by Huygens, 

 one of the leading names in the development of the undulatory 

 theory of light. In this diagram (Fig. 3) is represented in 

 section the different parts of the obstacle, c represents the 

 source of sound, B represents the flame, and A P q is the screen. 

 If we choose a point, p, on this screen, so that the whole dis- 

 stance from B to c, reckoned through P, viz. B P c, exceeds the 

 shortest distance B A c by exactly half the wave-length of the 

 sound, then the circular area, whose radius is A P, is the first 

 zone. We take next another point, Q, so that the whole dis- 

 tance B Q c exceeds the previous one by half a wave-length. 

 Thus we get the second zona represented by P Q. In like manner, 

 by taking different points in succession such that the last dis- 

 tance taken exceeds the previous one every time by half a 

 wave-length, we may map out the whole of the obstructing 

 screen into a series of zones called Huygens' zones. I have here 

 a material embodiment of that notion, in which the zones are 

 actually cut out of a piece of zinc. It is easy to prove that the 

 effects of the parts of the wave traversing the alternate zones are 



