ya}'.. 15, 1874J 



NATURE 



209 



the sound pulses ; they act precisely as this tube 

 does. 



As )ou know, light cannot well get round a corner ; 

 neither can sound, though it does so more easily than 

 light. This little bell acts automatically. I wind it up and 

 start it. At a few feet distance the flame answers to each 

 stroke. Placed behind a board, the flame becomes tranquil. 

 I again bring it out from behind the board, and the flame 

 jumps to each movement of the hammer. (For this ex- 

 periment the sensitive flame was arranged as in Fig. 4, 

 with a large glass funnel having its tubular end opposite 

 the root of the flame ; the board was held about 10 feet 

 distant from the mouth of the funnel.) Sound there- 

 fore can be shaded off in the same way that light 

 can be. 



In this bo.x, which is well padded, is a bell which I can 

 set ringing at pleasure. The only way by which the 

 sound can get out is this small square opening at one 

 side of it. The bell is now ringing without affecting the 

 sensitive flame (arranged as in Fig. 4) ; but when this box 

 is turned round, so that its opening faces the quiet flame, 

 we have it dancing and jumping as before. 



In other respects also there is a similarity between the 

 mode ot action of sound and light. 



When a beam from the electric lamp is allowed to fall 



upon the glass mirror in my hand, it is reflected from the 

 mirror, and the track of the beam being marked by the 

 dust floating in the room, you can see the direction which 

 it takes. This is in accordance with a well-known law, 

 namely, that the angle of incidence is equal to the angle 

 of reflection. It is perfectly plain to you that a line drawn 

 so as to fall at right angles upon this mirror would divide 

 that large angle formed by the two beams of light into 

 two equal angles. 



I hope now to make visible to your eyes the reflection 

 of sound in obedience to the same law. 



At one corner of the lecture table 1 place our sensitive 

 flame (b), at the opposite corner the padded box contain- 

 ing the electric bell {a) with its opening directed in the 

 path taken a moment ago by the beam of light, and I will 

 hold this board (<;), when everything is ready, where I 

 before held the glass mirror. My assistant will now set 

 the bell ringing. You observe that the flame is unin- 

 fluenced by it, but when I bring the board forward, the 

 shortening of the flame at each stroke of the bell, proves 

 that the law of the reflection of sound is the same as the 

 law of the reflection of light : the angle of incidence is 

 equal to the angle of reflection. In this case the flame is 

 knocked down by an echo. 



We have thus considered the reflection of sound from a 



plane surface ; let us now see if it behaves hke light 

 when reflected from plane surfaces. 



The beam of the electric lamp is now directed upon 

 the concave mirror. You can see the band of light 

 marked in the fine dust floating in the air ; as soon 

 as it strikes the polished surface it is thrown back, but 

 the rays no longer pursue parallel paths, they are con- 

 verged, thrown together into one spot. By holding a 

 piece of tracing paper at the point where they meet, 

 termed the focus, the brilliant little star of light caused 

 by their convergence is made visible. 



.Substitute for the lamp a small bell, and for the tracing 

 paper at the focus of the mirror our sensitive flame, and 

 the conditions are the same as in the previous experiment, 

 sound-w.aves taking the place of the waves of light. You 

 cannot see the track of these aerial pulses as you 

 could the luminous ones, but their obedience to the 

 same law of reflection is very manifest by the shortening 

 of the sensitive flame as each sound wave, reaches it. 

 The flame when out of the focus of the mirror is un- 

 affected ; replace it in the spot when the sound waves are 

 crowded together, and it responds to each stroke. Move 

 the bell so that the sound pulses, though only having the 

 same distance to travel to the flame, no longer fall on 

 the mirror : the flame remains perfectly quiet. 



We may go further still. Here are a pair of mirrors, 

 the curvature and size of which is the same. They are 

 arranged so as to face one another. A light is placed in 

 the focus of one, that its rays which fall divergent upon 

 the curved surface are reflected from it parallel, they 

 travel to the opposite mirror, and arc again converged ; a 



piece of tracing paper held at the focus of the farther 

 mirror shows the spot of light as before (Fig. 6). 



Sound is reflected in piecisely the same way, and the 

 sensitive flame when carefully manipulated can be used as 

 a means of proving this fact. For these experiments it is 

 essentially necessaiy that the flame be reduced to the 

 proper pitch of sensitiveness. By reducing the pressure 

 of the gas we can regulate the flame so that it will not 

 respond unless strongly agitated. The flame is placed in 

 the focus of the mirror {a), and when the bell is rung, not 

 being in the focus of the conjugate mirror, there is no 

 action. I now bring it into the focus (/') and the flame 

 shows a very strong action. 



By other modes of experimenting it has long been ascer- 

 tained that sound was thus reflected from plane and curved 

 surfaces ; but never before have these phenomena been 

 made visible. Hitherto these effects have been investi- 

 gated by the sense of hearing ; I have now been able to 

 prove them by appealing to your eyes. 

 {To be continued) 



SCHOLARSHIPS AND EXAMINATIONS FOR 

 NATURAL SCIENCE AT CAMBRIDGE, 1874 



THE following is a list of the Scholarships and Exhi- 

 bitions for proficiency in Natural Science to be 

 offered at the several Colleges in Cambridge during the 

 present year : — 



Trinity College. — One or more of the value of about 

 Sc/. per annum. The examination will commence 



