XXI. Photoqraphy of Somid- Waves by the " Schlieren- 

 Methode}' By R. W. Wood*. 



[Plates ni. & IV.] 



I HAVE always felt that the very beautiful optical method 

 devised in 1867 by Toepler f, for the study of "Schlieren," 

 or striae, is not as well known outside of Germany as it de- 

 serves to be, and trust that the photographs illustrating this 

 paper are sufficient excuse for bringing it before the readers of 

 the ' Philosophical Magazine.' Sound-waves in air were ob- 

 served by Toepler, but they have never to my knowledge been 

 photographed. When seen subjectively, the wave -fronts, if 

 at all complicated, cannot be very carefully studied, as they 

 are only illuminated for an instant, and appear in rapid 

 succession in different parts of the field of the viewing- 

 telescope. 



It occurred to me that if these waves could only be photo- 

 graphed, a most valuable set of lantern-slides could be pre- 

 pared for illustrating to students the changes in the form of 

 the wave-front, after undergoing reflexion, refraction, &c. 



In teaching the subject of optics we are compelled to 

 resort to diagrams when dealing with the wave-front, and the 

 student is apt to get the idea that it is merely a sort of 

 a conception, and that the ray is the real thing. I have 

 found these pictures of sound-waves very valuable in check- 

 ing this notion, showing as they do the bending of the 

 wave-front by refraction through a prism, the converging 

 wave from a concave reflector, the secondary wavelets formed 

 by transparent and opaque gratings, the diffraction produced 

 by the edge of an obstacle, and other phenomena connected 

 with wave-motion. The apparatus used for photographing the 

 waves is essentially the same as that which Toepler used for 

 viewing them, except that the illuminator has been improved 

 so as to give a much stronger light than the one originally 

 used, which was far too feeble to affect the photographic 

 plate. 



Fig. 1. 



b 



BBf^^Jpr ~ -^_~_ — " ~ ' 



il I ~° ~~ — —~~_~ 



The general arrangement of the " Schlieren " apparatus is 

 shown in fig. 1. A good-sized achromatic lens of the finest 



* Communicated by the Author, 

 t Wied. Ann. cxxxi. p. 33. 



