3()() PHOTOGRAPHY OF SOUND WAVES. 



Whon seen siil>j('ctiv(>ly. as was the caso in Too])!!'!"^ ('X])('riiii('iits, 

 tho wave fronts, if at all coniplicatcd, as they often arc, can not be 

 studied to advaiitag'e, tis they are illuminated for an instant only and 

 appear in rapid sueee.ssion in different parts of the field. By the aid 

 of photoo'raphy a pci'iuanent record of the forms can he ohtained and 

 .studied at leisure. The iirst series of photo^'raphs, pul)li>shed in the 

 Philosophical Maj>azine, were made with an apparatus similar to the 

 one to be })resently described; while most of those illustrating- this 

 article were made on a much larj;fer scale by employing a laro-e silvered 

 mirror in place of the lens, an improvement due to Professor Mach, of 

 Pi-at^ue. who has triven nuich attention to the subjcH't. 



As it is a matter of no trouble at all to set up in a fmv minutes, in 

 any physical laboratory, an apparatus for showing- the air waves sub- 

 jectively, and as the method does not seem to be as well known as it 

 deserves to be, a ))rief description of the " Schlieren " apparatus, as 

 Toepler named it, ma}' not be out of place. 



THE ArPAKATUS. 



The general ai-rangemcnt of the *" Schlieren "' apparatus is shown in 

 tigure I. A good-sized achromatic lens of the finest quality obtainable, 

 and of rather long focus, is the most important pai't of the device. 1 



sparH 



Fk;. 1. 



ha^ «' ))een using the object glass of a small ttdescope figured l)y the 

 late Alvan Clark. Its diameter is ij inches, and the focal length 

 about G feet. 1 have no doubt but that a smaller lens could be used 

 for viewing the waves, but one of at least this size is desirable for 

 photogriiphing them. 



The lens is mounted in front of a suitable source of light (in tne 

 present case an electric spark), which should l)e at such a distance that 

 its image on the other side of the lens is at a distance of about lo feet. 



The image of the spark, which we will suppose to l)e straight, hori- 

 zontal, and very narrow, is about two-thirds covered w itli a horizontal 

 diaphragm (<-/). and immediately behind this is placed the viewing tele- 

 scope. On looking into the telescope we see the field of the lens uni- 

 fonnly illuminated by the light that passes under the diaphragm, since 

 every part of the image of the spark receives light from the whole 

 lens. If the diaphragm be lowered, the field will darken ; if it l)e raised, 

 the illumination will be increased. In general it is best to have the 

 diaphragm so adjusted that the lens is quite feebly illuminated, though 

 this is not true for photographic work. Let us now suppose that there 

 is a globular mass of air in front of the lens of slightly greater optical 



