116 Mr. G. H. Berry on 



and vertical deflexion was obtained in the following way : — 

 A 6 lb. weight was attached to the middle o£ the string by a 

 thin cord passing over a pulley and pulled the string towards 

 the sound- board. The movement of the bridge was measured 

 with a vernier microscope. Half the weight or 3 lbs. may 

 be considered as acting on the free bridge. The observed 

 mean deflexions of the free bridge were 0*060 cm. at right- 

 angles to the string and 0'003 cm. parallel to the string. 

 When a force is applied at the middle of the string and at 

 right-angles to it, the corresponding movement of the free 

 bridge is therefore 20 times greater in the direction at right- 

 angles to the string, or horizontally, than it is vertically. 

 It would therefore seem that the horizontal motion of the 

 bridge setting the sound-board in vibration, as in (3), is likely 

 k) be much greater than the motion of the bridge parallel 

 to the string and setting the sound-board in vibration as 

 in (1) and (2). 



Photographic Curves. 



The next step was to obtain photographic time-displacement 

 curves by means of the optical lever. 



A beam of light from an electric arc passes through a 

 narrow vertical slit and falls upon a small concave mirror 

 mounted on a tiny three-legged table. One leg of the table 

 is connected to the face of the bridge, and the other two 

 rest in a hole and slot respectively on an independent support. 

 The table is held in place by elastic bands. Any movement 

 of the bridge at right-angles to the string causes the beam 

 of light reflected by the mirror to vibrate horizontally. The 

 image of the slit is focussed on a strip of photographic film 

 fastened round the drum of an ordinary phonograph. A 

 horizontal slit is fastened immediately in front of the film, 

 this allows a small square of light only to fall on the film. 

 In front of the film is also a 1/1 plate shutter with pneumatic 

 release. To obtain the photograph of the vibrations of the 

 bridge parallel to the string the optical lever is mounted 

 with one leg attached to the lower side of the bridge, and 

 the vibrations of the beam of light being then vertical, the 

 drum of the phonograph and the two slits have to be arranged 

 accordingly. 



The distance of the leg of the optical lever attached to the 

 bridge from the line joining the other two legs was 0*35 cm., 

 and the distance between the mirror and the film was 

 127 cm. Thus the magnification would be 360. 



The speed of the film was 67*7 cm. per second. 



The string used was 99 cm. long between the bridges, was 



