32 



NATURE 



{May II, 1876 



held in the hand close to the head. The screw a, Fig. I, 

 must be adjusted to give the best efifect. 



The other receiving instrument is the most interesting 

 of the two. It consists of a small induction coil used in 

 conjunction with a peculiar sounding-box, as shown in 



Fig. 3- 



Here the line-current is passed to earth through the 

 primary circuit P of the small induction coil, and the in- 

 duced current is led to the sounding-box. This consists 

 of a flat hollow cylindrical wooden box B, covered by a 

 convoluted face of sheet zinc with two air holes hh, per- 

 forated in it, this box is attached to a metal axle A, 

 turning in forked iron bearings, insulated from but sup- 

 ported by an iron stand S. By this means the sounding- 

 box can be revolved by the ebony handle E. The zinc 

 face is connected across the empty interior of the box by 

 a wire w to the metal bearings on the other side. One 

 end of the secondary circuit of the induction coil is to be 

 connected to the metal bearing by the terminal a, and 

 the other to a short bare wire held in the left hand. On 

 then striking a finger of the hand holding the wire smartly 

 across the zinc face, the proper note is sounded by the 

 box ; or, what is more convenient, on turning the box by 

 the insulated handle and keeping the point of the finger 

 rubbing on its face, the note is heard. The rough under 

 side of the finger pressed pretty hard on the bulging part 

 of the face is best. The instant the current is put on by 

 the sending key K, Fig. i, the dry rasp of the skin on 

 the zinc-surface becomes changed into a musical note. 



These " sounders " can be made to receive indifferently 

 a variety of notes. I have under my care at present a 

 telephone with four, transmitters tuned to give the four 

 notes of the common chord, and two receivers, which 

 interpret equally well any one of these notes or all 

 together. But sounders are also made in the same way 

 which will emit only one special note, and so are sensible 

 only to the corresponding current. It is by their means 

 that the telephone can be applied to multiplex telegraphy. 

 As many as eight transmitters may be set to interrupt the 

 line current according to the vibrations of eight different 

 tuning-forks, and the resultant current can be made by 

 means of eight special receivers to reproduce the same 

 number of corresponding notes at the distant station. 

 The current is controlled by eight keys at the sending 

 end and sifted by eight sounders at the receiving end, 

 each sounder being sensitive only to those portions of the 

 current affected by its corresponding transmitter. The 

 superimposed effect of the eight keys and transmitters on 

 the hne current can all be separately interpreted at the 

 receivmg end. Thus eight messages might be trans- 

 mitted simultaneously along one wire in the same direc- 

 tion. It would seem hitherto, however, that this method 

 of telegraphy by the telephone is inferior to the ordinary 

 ''methods in point of speed of signalling, and in the 

 length of circuit which can be worked by a given battery 

 power. J. MUNRO 



OUR PERCEPTION OF THE DIRECTION OF 

 A SOURCE OF SOUNDS 



THE practical facility with which we recognise the 

 situation of a sounding body has always been rather 

 a theoretical difficulty. In the case of sight a special 

 optical apparatus is provided whose function it is to 

 modify the uniform excitation of the retina, which a 

 luminous point, wherever situated, would otherwise pro- 

 duce. The mode of action of the crystalline lens of the 

 eye is well understood, and the use of a lens is precisely 

 the device that would at once occur to the mind of an 

 optician ignorant of physiology. The bundle of rays, 

 which would otherwise distribute themselves over the 

 entire retina, and so give no indication of their origin, are 



' Abstract of a Communication to the Musical Association, by Lord 

 Rayleigh, F.R.S. 



made to converge upon a single point, whose excitation is 

 to us the sign of an external object in a certain definite 

 direction. If the luminous object is moved, the fact is at 

 once recognised by the change in the point of excitation. 



There is nothing in the ear corresponding to the 

 crystaUine lens of the eye, and this not accidentally, so to 

 speak, but by the very nature of the case. The efficient 

 action of a lens depends upon its diameter being at least 

 many times greater than the wave-length of light, and for 

 the purposes of sight there is no difficulty in satisfying 

 this requirement. The wave-length of the rays by which 

 we see is not much more than a ten-thousandth part of 

 the diameter of the pupil of the eye. But when we pass 

 to the case of sound and the ear the relative magnitudes 

 of the corresponding quantities are altogether different. 

 The waves of sound issuing from a man's mouth are about 

 eight feet long, whereas the diameter of the passage of the 

 ear is quite small, and could not well have been made a 

 large multiple of eight feet. It is evident therefore that 

 it is useless to look for anything corresponding to the 

 crystalline lens of the eye, and that our power of telling 

 the origin of a sound must be explained in some diflferent 

 way. 



It has long been conjectured that the explanation turns 

 upon the combined use of both ears ; though but little 

 seems to have been done hitherto in the way of bringing 

 this view to the test. The observations and calculations 

 now brought forward are very incomplete, but may perhaps 

 help to clear the ground, and will have served their pur- 

 pose if they induce others to pursue the subject. 



The first experiments were made with the view of find- 

 ing out with what degree of accuracy the direction of a 

 sound could be determined, and for this it was necessary 

 of course that the observer should have no other material 

 for his judgment than that contemplated. 



The observer, stationed with his eyes closed in the 

 middle of a lawn on a still evening, was asked to point 

 with the hand in the direction of voices addressed to him 

 by five or six assistants, who continually shifted their 

 position. It was necessary to have several assistants, 

 since it was found that otherwise their steps could be 

 easily followed. The uniform result was that the direc- 

 tion of a human voice used in anything like a natural 

 manner could be told with certainty from a single word, 

 or even vowel to within a few degrees. 



But with other sounds the result was different. If the 

 source was on the right or the left of the observer, its 

 position could be told approximately, but it was uncertain 

 whether, for example, a low whistle was in front or behind. 

 This result led us to try a simple sound, such as that 

 given by a fork mounted on a resonance box. It was 

 soon found that whatever might be the case with a truly 

 simple sound, the observer never failed to detect the 

 situation of the fork by the noises accompanying its 

 excitation, whether this was done by striking or by a 

 violin bow. It was therefore necessary to arrange the 

 experiment differently. Two assistants at equal distances 

 and in opposite directions were provided with similar 

 forks and resonators. At a signal given by a fourth, both 

 forks were struck, but only one was held over its resona- 

 tor, and the observer was asked to say, without moving 

 his head, which he heard. When the observer was so 

 turned that one fork was immediately in front and the 

 other immediately behind, it was impossible for him to 

 tell which fork was sounding, and if asked to say one or 

 the other, felt that he was only guessing. But on turning 

 a quarter round, so as to have one fork on his right and 

 the other on his left, he could tell without fail, and with 

 full confidence in being correct. 



The possibility of distinguishing a voice in front from a 

 voice behind would thus appear to depend on the com- 

 pound character of the sound in a way that it is not easy 

 to understand, and for which the second ear would be of 

 no advantage. But even in the case of a lateral sound 



