616 Mr. C. V. Raman on 



of the string, and might, under suitable circumstances, be 

 expected to oscillate with the double frequency. To verify 

 this point, I did, at that time, think only of direct aural 

 observation on a specially constructed model. This idea was 

 worked out by me immediately and with success. To hear a 

 note of the double frequency it was essential that all sounding- 

 parts that would emit the fundamental should be abolished. 

 In other words, the model sonometer (for so it was) had 

 only, so to speak, a very much magnified bridge, i. e., only a 

 sounding-board normal to the wire, instead of, as usual, one 

 parallel to it. This was arranged without difficulty. The 

 sounding-board was fixed in a rigid frame ; one end of the 

 stretched wire was attached to the frame, and the other end 

 normally to the centre of the sounding-board. It was 

 verified by comparison with a sonometer of the ordinary 

 type that the note emitted by the instrument had double the 

 frequency of the vibrations of the wire, in whatever way 

 the latter was set in vibration. A better musical effect was 

 obtained when the sounding-board was replaced by a mem- 

 brane stretched on a circular ring. 



The frequency-relation was also verified by the following 

 (it is believed novel) method. A point on the vibrating 

 wire quite close to the sounding-board has only a microscopic 

 motion, This has, however, two components : one transverse 

 to the wire having the same frequency as its oscillation ; the 

 other normal to the sounding-board, i.e., lengthwise of the 

 wire, having double the frequency of the vibrations of the 

 latter. The path described is the characteristic Liissajous's 

 figure, i. e. a parabolic arc. This was verified by observation. 



For photographing the vibration-curves of the wire and 

 the sounding-board in my apparatus, a three-legged optical 

 lever was as usual employed. The amplitude of oscillation 

 of the sounding-board was not. however, so small as to 

 require very considerable magnification. One leg rested 

 upon the edge of the sounding-board, the other two in a 

 hole and a slot cut in a brass plate kept fixed nearly flush 

 with the sounding-board. The lever had only a plane 

 mirror attached to it. One source of light was a horizontal 

 slit, and the other was a vertical slit placed immediately 

 behind the oscillating wire. Both were illuminated by sun- 

 light and had collimating lenses in front of them. The 

 light from the former after reflexion at the oscillating 

 mirror, and the light from the latter after reflexion at a 

 fixed mirror, fell upon the lens (having an aperture of about 

 2 inches diameter) of a roughly constructed camera. In 

 the focal plane of the latter was placed a brass plate with a 



