Photography of Sound-waves, &c. 



283 



Photography of Sound-waves, and the Kinematographic Demon- 

 stration of the Evolutions of Keflected Wave-fronts." By 

 E. W. Wood, Assistant Professor of Physics in the Univer- 

 sity of Wisconsin. Communicated by C. V. Boys, F.E.S. 

 Eeceived February 10, — Eead February 15, 1900. 



In a paper pubhshed in the ' Philosophical Magazine ' (August, 1899) 

 I gave an account of a series of photographs of sound-waves under- 

 going reflection, 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 apparatus, a very 

 much better and" more complete series of photographs; and at the risk 

 of subjecting 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 rays 

 issuing from a point, is very beautifully shown, the spherical wave 

 diverging from one focus being transformed by reflection into a con- 

 verging sphere which shrinks to a point at the other focus. 



The transformation of a spherical into a plane wave by a parabolic 

 mirror is also well shown (fig. 1). 



Fig. 1. 



The effect of spherical aberration of circular mirrors is beautifully 

 exhibited in several cases. 



When a plane wave enters a hemispherical mirror the reflected wave 

 front is cusped, and the cusp will be seen to lie always on the caustic 

 surface. The form of the complete wave in this case is not unlike a 



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