Oct. 20, 1 881] 



NATURE 



587 



placed wherever there is room for them, generally under- 

 neath the stage ; but as they would become too highly 

 polarised if left in circuit during a whole representation, it 

 is necessary to renew them every quarter of an hour, a.nd 

 for that purpose a commutator lias been provided which 

 allows the change to be made in a moment. This comu- 

 tator consists of a board furnished with as many spring- 

 plates as there are transmitters, and serves to keep up the 

 connection between the transuutters and the batteries. 



The greatest difficulty that has been met with has been 

 how to render the transmitter more sensitive to the voices 

 of singers than to the loud sounds emitted from the 

 instruments of the orchestra, which would otherwise pre- 



dominate. M. Ader has had to make a number of 

 acoustic studies and trials on this point, which we cannot 

 explain without going too much into detail, and has com- 

 pletely vanquished the difficulty ; we will, however, 

 explain the means which M. Ader has employed to 

 enable the auditor to follow to some extent the movements 

 of the actors on the stage. 



Everybody knows the stereoscope, which enables a 

 person, by means of the superposed visual impressions of 

 the two eye?, to see the stereoscopic images with their 

 natural relief. M. .-Xder applies the same principle to the 

 perception of sounds. 



Suppose two microphonic transmitters, placed on the 



stage at T and t' (Fig. i). Let one of them be connected 

 by wires with the receiver R, and the other with the 

 receiver R', these receivers being applied to the two ears 

 of the auditor, and suppose an actor to stand first at A 

 and then at a'. In the former position, as he is nearer to 

 the transmitter T than to t', his singing will be heard 

 loudest with the left ear ; but when he is at a' he is nearer 

 to t' than to T, and the right ear will receive the strongest 

 impression. Thus as he goes from A to a' the definite 

 sensation which the auditor will receive will be that of a 

 diminution of loudness in the left and an increase of 

 loudness in the right ear, which is the same as the sensa- 

 tion which we experience when a person who is speaking 



walks from our left to our right. The same principle will 

 apply to a number of actors crossing one another on the 

 stage. Fig. 2 explains how this idea has been carried out 

 at the Opera House. 



We have already stated that there are five transmitters 

 on each side of the prompter's bo.x [marked soziffleicr in 

 the figure] along the edge of the stage. Each of these 

 transmitters has its own separate circuit, and conse- 

 quently its own separate underground cabL*. On arriving 

 at the' hearing room, the cables terminate each in eight 

 receivers, but aUvays in such a manner that to each 

 auditor the effects are very different for his two ears. 

 Fi'^. 2 shows the course of the circuits for two trans- 



