226 Photography of Sound- Waves by the u Schlieren-Methode." 



in fig. 13, the secondary wavelets in No. 3 of the series being 

 especially beautiful. 



Figs. 12 and 13 are interesting in their connexion with 

 reflexion-gratings. 



The formation of a musical note by the reflexion of a single 

 pulse from a flight of steps is shown photographed in fig. 14. 

 This phenomenon is often noticed on a still night when 

 walking on a stone pavement alongside a picket-fence, the 

 sound of each footstep being reflected from the palings as a 

 metallic squeak, which Young has pointed out to be analogous 

 to the power of a diffraction-grating to construct light of 

 a definite wave-length. 



In No. 1 of the series the wave has just reached the top 

 step of the flight. In No. 2 it is half-way down, and we 

 begin to see traces of the secondary wavelets, while in No. 3 

 it has reached the bottom, and we see a beautiful train of 

 little reflected waves, constituting a musical note of very high 

 pitch. The wave-length of the reflected disturbance is of 

 course twice the height of the steps, or very nearly. In 

 No. 4 the original wave is out of the field, and there remains 

 only the train. 



The passage of one sound-wave through another is shown 

 in fig. 15. Here we have two spark-gaps arranged in series 

 before the lens, so that two waves are started simultaneously. 

 The wave-fronts are seen to have passed through each other 

 without suffering any disturbance whatever. 



The condensation in these waves must be very considerable, 

 since they show on the photographic plate even when the 

 apparatus is not made very sensitive, that is, wlien a con- 

 siderable part of the light passes below the diaphragm. 

 Under these circumstances it is impossible to see them with 

 the telescope, or only with the greatest difficulty. 



In conclusion a few words regarding the apparatus may 

 be helpful to those wishing to repeat the very beautiful 

 experiments of Toepler. An induction-coil capable of giving 

 a three or four inch spark is about right, while a good-sized 

 test-tube partly filled with mercury, and standing in a 

 cylinder of mercury, will be found most convenient for a 

 leyden-jar. The balls between which the sound-spark passes 

 should be adjusted so as to obtain almost the maximum spark 

 possible, which will in general be rather less than half what 

 the coil will give between its terminals. I find that the best 

 results are obtained when the sparks give off the same crackle 

 found desirable in experiments with Hertz waves. Fresh 

 plates of glass should be put in the illuminator every little 

 while. 



