356 POPULAR SCIENCE MONTHLY. 



was that of a strong, black, circular line, opening out with terrific speed 

 from the point of explosion as a center. It was impossible to judge 

 of the thickness of the shadow; it may have been three feet, or it may- 

 have been more at first, and have gradually become less in thickness, 

 or possibly in depth of shade." 



Unfortunately, Professor Boys's apparatus did not work satisfac- 

 torily, but a most interesting series of pictures was secured by the kine- 

 matograph. This instrument had been constructed especially for taking 

 ipctures at a very high rate of speed, viz.. eighty exposures a second, or 

 four times the usual number. The sound wave appears in the first 

 dozen pictures as a hazy ring of light, opening out from the center of 

 explosion. The ring, though not very conspicuous when the pictures 

 are viewed singly, becomes a striking object when they are projected in 

 rapid succession on the screen. We see the rush of smoke along the 

 ground to the box in which the explosion is confined (the smoke of the 

 quick fuse); then comes the burst of the explosion with such startling 

 realitythat we involuntarilyjump. The image of the sound wave flies out 

 in the form of a white ring, and is gone in a moment; and there remain 

 only the rolling clouds of smoke. It is interesting to observe the de- 

 velopment of the explosion by running the machine quite slowly, and by 

 thus magnifying time to follow the changes which ordinarily occur in 

 such rapid succession that the eye is unable to perceive them. 



Of this series of pictures, Professor Boys says: •■The kinematograph 

 fails to show any black ring; and this is not surprising, as with the 

 exposure of about one one hundredth of a second the shadow would have 

 to be at least eleven feet thick in order that some part should remain 

 obscured during the whole exposure. As a fact, there is clearly seen 

 a circular light shading, which does — so far as one can judge from the 

 supposed rate of working and the known distances — expand at about 

 the same rate as the observed shadow, but it is lighter than the ground 

 and shaded, instead of being dark and sharp, as seen by the eye." 



So much for the visibility of sound under ordinary conditions. In 

 the laboratory, by means of an optical contrivance due to the German 

 physicist Toepler, we can secure a means of illumination so sensitive 

 that the warm air rising from a person's hand appears like dense black 

 smoke. Moreover, since we are working on a small scale, we can use 

 the electric spark as the source of light, and dispense with the photo- 

 graphic shutter. This is a great advantage, for the time of the exposure 

 is, under these conditions, only about one fifty-thousandth of a second, 

 during which time the sound wave will move scarcely a quarter of an 

 inch. During the past year I have made a very complete series of 

 photographs of sound waves, which illustrate in a most beautiful man- 

 ner the fundamental principles of wave motion. It is not practicable to 

 give here a full description of the apparatus used, but a brief outline may 



