76 M. C. Sondhauss on the Be fraction of Sound. 



Distance of the watch from Distance of the ear from 

 the centre of the lens. the centre of the lens, 

 ft. in.. ft. 



8 10 



1 10 



13 9 



16 8 



2 6 



2 9 4 



3 3 



3 6...... 2 



4 2 



6 2 



In the two last observations the lens had become flaccid by 

 the escape of carbonic acid, and was filled again by the blowing 

 in of air. Hence the refraction is less than in the foregoing 

 observations. 1 would remark, that, where the watch was brought 

 more closely to the lens, so that its distance amounted to about 

 one or two feet, its ticking was also audible immediately behind 

 the lens, and from this forward to the distance given above. 

 Where, however, the watch was placed at a distance of from four 

 to five feet from the lens, I could hear no sound immediately 

 behind the lens, but heard it plainly when the ear was placed at 

 a distance of one or two feet from the latter. In the last four 

 observations the ticking was heard most distinctly when the ear 

 was 1' 3" distant from the centre of the lens. 



The distance of the point of convergence for parallel rays of 

 sound, or the focal distance of the collodion lens, could not, for 

 the reasons before given, be obtained ; but judging from the expe- 

 riments hitherto made, it cannot be much beyond a foot. Accord- 

 ing to this, the index of refraction for sound in its passage from 

 air into carbonic acid must be somewhat greater than the ratio 

 of its velocities in these two media. If we assume, in accordance 

 with the experiments of Dulong, the velocity of sound in air to 

 be 333 metres, and in carbonic acid 261*6 metres, and regard 

 the ratio of both, n= 1*272, as the index of refraction of the 

 carbonic acid lens, the radii of the two spherical collodion mem- 

 branes amounting to about 8"* 5, we obtain, according to the 

 known formula for the focal distance of glass lenses. 



7=(»-^)(i4> 



for the focal distance of parallel rays of sound refracted by my lens, 



/=15"*6, 



