THE ANALOGY OF SOUND AND LIGHT. 99 



tions of the rate at which each travels, he gave this as a very intelligible 

 one : If a canon were fired in London the sound would take about eight 

 minutes to travel to Birmingham, a little over one hundred miles, while in 

 the same time the light from the flash would have traveled to the sun, a 

 distance of over ninety millions of miles. But, though they so differ in the 

 rate of progress, both light and sound show many phenomena in common. 



•'In the experiments made during the evening the sensitive flame was 

 used as a detector of sound. This delicate acoustic reagent, familiar to 

 London audiences through Professor Tyndall's lectures, was first, we be- 

 lieve, discovered in 1866, by Professor Barrett, though he modestly did not 

 allude to the fact. Indeed, most of the experiments shown during the 

 evening formed the subject of a paper read by him before the Eoyal Dublin 

 Society in January, 1868, and the discovery of the ratios referred to at the 

 end of the lecture was announced in the Quarterly Journal of Science for 

 1870. The performance of the experiments, however, was entirely new to 

 a London audience. 



"The analysis of the phenomena of light and sound were illustrated in 

 the following order : 1. Both light and sound get feebler as they leave 

 their source of origin. In the case of sound this was shown with a loud 

 ticking watch and a sensitive flame. 2. In reflection the angle of incidence 

 is the same as the angle of reflection. In the case of sound, this was shown 

 with the sound of a whistle sent along a tube, and reflecting along another 

 placed at an angle to it from a reflector placed at the end where they ap- 

 proached. The distance to which a feble sound might be reflected percept- 

 ibly from a concave mirror was shown with mirrors over thirty feet apart. 



3. With refraction, in the case of light, familiar convex lenses were used • 

 and in the case of sound, analogous but less familiar lenses of gas of a dif- 

 ferent density from air were used. A collodion balloon, filled with carbonic 

 acid gas, served as a double convex lens, and its action was manifested by 

 the concentration of sound from the ticking^watch on to the sensitive flame. 



4. Both light and sound suffer absorption in 23assing through non-homoge- 

 neous media. Professor Tyndall's apparatus, showing the 'echoing back' of 

 sound in passing through successive alternating layers of gas of different 

 densities, is now well known, and every one is familiar with the fact that, 

 though light may traverse a vessel of clear water, it can no longer travel 

 when it is filled with bubbles of transparent air. 5. There is an analogy 

 between the sympathy among the same notes of a gamut and the sympathy 

 among individual colors in the spectrum. An incandescent body that pro- 

 duces a particular bright band in the rear of the spectrum will, when in a 

 gaseous state, absorb light, and cause a dark band in exactly the same part 

 of the scale. Tuning forks, wires, or columns of air in jars are responsive 

 to vibrations produced by others exactly in unison, but only to those. This 

 was shown in various ways in a very clear manner. 6. An analogy, which 

 Professor Barrett called a more fanciful one, was spoken of. All the com- 

 plex music of an orchestra is the result of a few simple notes variously 



