24 SENSITIVE FLAMES AND SOUND-SHADOWS. 



grating is made is beyond the scope of the present paper.* As- 

 suming its use, the sensitive flame enables us to detect a focal 

 area of noise, at which the flame is violently agitated, and around 

 this are alternate rings of silence and fainter noise diminishing 

 in strength with increase of distance from the central focus. 



By admitting light through two small openings close together, 

 the waves coming from a distant bright point and hence reaching 

 the two openings in the same phase, hyperbolic lines of interfer- 

 ence like those shown in Fig. 8 were traced in space by Fresnel. 

 The writer has recently done the same with sound-waves, using 

 the sensitive flame as an explorer. Bands of alternate noise 

 and silence have in like manner been traced by him in air, pro- 

 duced by interference between the waves x>i'oceeding directly 

 from the whistle and those reflected from a smooth surface placed 

 horizontally on the table. 



The wave theory of sound has long been impregnable; but 

 these beautiful analogies between light and sound, though pro- 

 vided for by theory, have been experimentally demonstrated only 

 recently. Such new and unexpected confirmations, new points of 

 contact, are always welcome, even though they be not needed for 

 the establishment of a theory. They are the results of prevision 

 based on the assumption that an elastic material medium is 

 needed for the propagation of sound, and are wholly inexplicable 

 on any theory of emanation analogous to Newton's emission the- 

 ory of light. 



* For this explanation the reader is referred to an article on " Diffraction of Sound," in 

 the "Journal of the Franklin Institute," for June, 1889. 



