148 Prof. R. W. Wood on Photography of Sound- Waves 



break is quickened. This is best seen with carbon, less distinctly 

 with copper, while with platinum-platinum and platinum- 

 mercury breaks the best capacity and the maximum spark- 

 length appear to be unaffected by the quickness of the break. 

 But the behaviour of zinc is altogether anomalous; for with 

 this metal an increase in the quickness of the break is accom- 

 panied by an increase in the best capacity, and, moreover, the 

 way in which the curves of fig. 6 intersect each other shows 

 that for certain capacities a quick break may actually give a 

 shorter spark than a slow break. This peculiar behaviour of 

 zinc when used as a break-pole is probably connected in 

 some way with its non-arcing properties. 



IX. Photography of Sound-ivaves, and the Kinematographic 

 Demonstration of the Evolutions .of Reflected Wave-fronts. 

 By R. W. Wood, Assistant Professor of Physics in the 

 University of Wisconsin *. 



IN a paper published in the ' Philosophical Magazine ' 

 (August 1899) I gave an account of a series of photo- 

 graphs of sound-waves undergoing reflexion, refraction, 

 diffraction, &c, which were made chiefly for the purpose of 

 illustrating certain optical phenomena to classes. 



The waves were in every case single pulses in the air 

 produced by electric sparks, illuminated and photographed 

 by the light of a second spark, properly timed with reference 

 to the first, the apparatus being essentially the same as that 

 employed by Toepler for the study of striae. 



I have recently secured, by means of an improved appa- 

 ratus, a very much better and more complete series of 

 photographs. A silvered mirror of eight inches aperture was 

 used instead of the achromatic lens. This arrangement was 

 used by Mach for a similar purpose, and has certain ad- 

 vantages. Instead of allowing a torrent of sparks to pass 

 between the terminals and receiving the images on a moving 

 plate, as before, single sparks were used and the plate 

 advanced a sufficient distance after each discharge. This 

 eliminated the hot-air currents from the pictures entirely, 

 the heated air not having time to rise from behind the ball 

 terminal, before the picture is taken. At the risk of sub- 

 jecting myself to criticism for bringing matter already 

 published to the attention of the Society, I wish to devote a 

 few minutes to a very rapid inspection of them. 



The following cases, that were not represented in my first 

 paper, I think I may safely comment upon. 



The conjugate foci of the elliptical mirror, aplanatic for 



* Communicated by the Author, having been read before the Royal 

 Society, Feb. 15, 1900. 



