Objective Reality of Combination Tones, 345 



circles of holes on the siren appear stationary when viewed 

 through the slit, if the velocity of rotation was such that the 

 desired note or notes would be given by the same or other 

 circles of holes. An observer watching the siren through the 

 slit and pressing lightly on the axis with a straw, was able to 

 adjust its speed so that the required combination of sounds 

 was produced for several, and in some cases for many seconds 

 at a time. 



We have also projected the image of the row of holes on a 

 screen, the cone of light passing at its narrowest part through 

 the slit in the tinfoil screens carried by the auxiliary fork. 

 When the upper siren was to be used, two semicircles of light 

 mirror-glass were laid on the disk of the lower siren to which 

 the two halves of a paper cog-wheel were fastened. The 

 image of the cogs could thus be projected. 



The third method of determining the speed of the siren 

 depended on the use of an instrument first devised by 

 Lord Ray lei gh in 1880. A mass of air enclosed in a tube is 

 excited by resonance, and the fact of the excitation is indicated 

 by a light mirror set where the motion is greatest, and inclined 

 at 45° to the direction of the air- currents. In accordance 

 with the general law that a lamina tends to place itself per- 

 pendicular to the direction of a stream, the mirror moves 

 when the air vibrates. In the original apparatus the amount 

 of the movement was controlled by magnets. Since that 

 date Professor Boys has modified the instrument by substi- 

 tuting a quartz-thread suspension for a silk fibre, and using 

 the torsion of the thread instead of the directing force of the 

 magnets. He exhibited the apparatus during a lecture 

 delivered before the British Association in Leeds. It may be 

 called a mirror-resonator. 



Professor Boys has been good enough to make two of these 

 instruments for us, and it was decided that one of them should 

 respond to 161 vibrations per second. The movement of the 

 spot of light reflected from the mirror informed us when 

 the siren was giving this note, and this fact was utilized 

 in one of the experiments. 



Sensitiveness of the Apparatus, 



The results were in general improved by laying a small 

 strip of dry blotting-paper upon the prongs. Mere accidental 

 disturbances died out more quickly, and the vibrations of the 

 fork diminished more rapidly, when the notes wdiich had pro- 

 duced them ceased to sound. 



A forced movement could be produced in the fork by 



