342 



NATURE 



[August 9, 19CXD 



THE PHOTOGRAPHY OF SOUND-WAVES 

 AND THE DEMONSTRATION OF THE 

 EVOLUTIONS OF REFLECTED WAVE- 

 FRONTS WITH THE CINEMATOGRAPH. 



Introduction. 



IN a paper published in \.\i& Philosophical Magazine 

 for August 1899, I gave an account of some experi- 

 ments on the photography of sound-waves, and their 

 apphcation in the teaching of optical phenomena. Since 

 writing this paper I have extended the work somewhat 

 and at a meeting of the Royal Society on February 15, 

 1900, gave an account of this work, and demonstrated 

 certain features of wave motion with the cinematograph. 



In the present article I propose to give a somewhat 

 more extended account of the work, paying especial 

 attention to the analogies between the sound-waves and 

 waves of light. 



In teaching the subject of optics we are obliged to 

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

 in spite of all that we can do, the student is apt to form 

 the opinion that the rays are the actual entities, and that 

 wave-fronts are after all merely conceptions. 



The set of photographs illustrating this article will, I 

 think, be of no small use to teachers in ridding the minds 

 of students of the obnoxious rays, and impressing the 

 fact that all of the common phenomena of reflection, 

 refraction and diffraction are due simply to changes 

 wrought on the wave-front. 



Sound-waves in air were first observed and studied by 

 Toepler, by means of an exceedingly sensitive optical 

 contrivance for rendering visible minute changes 

 in the optical density of substances. A very 

 full description of the device will be found in 

 Toepler's article ( Wied. Annalen^ cxxxi.), while 



a brief account of it will be given presently. t~\^ (h 



. The waves in question are the single pulses «^V^^pii/ 

 of condensed air given out by electric sparks. lol 



A train of waves would complicate matters too 

 much, and for illustrating the optical phenomena 

 which we are to take up would be useless. 



The snap of the spark gives us just what we require, 

 namely, a single wave-front, in which the condensation 

 is considerable. 



When seen subjectively, as was the case in Toepler's 

 experiments, the wave-fronts, if at all complicated, as 

 they often are, cannot be studied to advantage, as they 

 are illuminated for an instant only, and appear in rapid 

 succession in different parts of the field. By the aid of 

 photography a permanent record of the forms can be 

 obtained and studied at leisure. The first series of 

 photographs, published in the Philosophical Magazine^ 

 were made with an apparatus similar to the one to be 

 presently described ; while most of those illustrating this 

 article were made on a much larger scale by employing 

 a large silvered mirror in place of the lens, an improve- 

 ment due to Prof. Mach, of Prague, who has given much 

 attention to the subject. 



As it is a matter of no trouble at all to set up in a i&^ 

 minutes, in any physical laboratory, an apparatus for 

 showing the air-waves subjectively, and as the method 

 does not seem to be as well known as it deserves to be, a 

 brief description of the " Schlieren " apparatus, as Toepler 

 named it, may not be out of place. 



The Apparatus. 

 The general arrangement of the " Schlieren " appa- 

 ratus is shown in Fig. i. A good-sized achromatic lens 

 of the finest quality obtainable, and of rather long focus, 

 is the most important part of the device.. I have been 

 using the object-glass of a small telescope figured by the 

 late Alvan Clarke. Its diameter is five inches, and the 

 focal length about six feet. I have no doubt but that a 

 smaller lens could be used for viewing the waves, but 



NO. 1606, VOL. 62] 



one of at least this size is desirable for photographing 

 them. 



The lens is mounted in front of a suitable source of 

 light (in the present case an electric spark), which should 

 be at such a distance that its image on the other side of 

 the lens is at a distance of about fifteen feet. 



The image of the spark, which we will suppose to be 

 straight, horizontal, and very narrow, is about two- 

 thirds covered with a horizontal diaphragm (<:i), and im- 

 mediately behind this is placed the viewing-telescope. 

 On looking into the telescope we see the field of the lens 

 uniformly 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 be 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 photo- 

 graphic work. Let us now suppose that there is a 

 globular mass of air in front of the lens of slightly greater 

 optical density than the surrounding air {b). The rays of 

 light going through the upper portion of this denser mass 

 will be bent down, and will form an image of the spark 

 below the diaphragm, allowing more light to enter the 

 telescope from this particular part of the field ; conse- 

 quently, on looking into the instrument, we shall see the 

 upper portion of the globular mass of air brighter than 

 the rest of the field. The rays which traverse the under 

 part of " b" however, will be bent up on the contrary, 

 forming an image of the spark higher up, and wholly 

 covered by the diaphragm ; consequently this part of the 



spark 



field will appear black. It will be readily understood 

 that, with the long path between the lens and the image 

 a very slight change in the optical density of any portior> 

 of the medium in front of the lens will be sufficient 

 to raise or depress the image above or below the edge 

 of the diaphragm, and will consequently make itself 

 manifest in the telescope. 



The importance of using a lens of first-class quality is 

 quite apparent, since variations in the density of the glass 

 of the lens will act in the same way as variations in the 

 density of the medium before it, and produce unequal 

 illumination of the field. It is impossible to find a lens 

 which will give an absolute even, feeble illumination, but 

 a good achromatic telescope objective is perfect enough 

 for every purpose. A more complete discussion of the 

 operation of the apparatus will be found in Toepler's 

 original paper in the Annalen. The sound waves, which 

 are regions of condensation, and consequent greater 

 optical density, make themselves apparent in the same 

 way as the globular mass of air already referred to. 

 They must be illuminated by a flash of exceedingly short 

 duration, which must occur while the wave is in the field 

 of view. 



Toepler showed that this could be done by starting the 

 sound-wave with an electric spark, and illuminating it 

 with the flash of a second spark occurring a moment 

 later, while the wave was still in the field. A diagram of 

 the apparatus used is shown in Fig. 2. In front of the 

 lens are two brass balls (a, a), between which the spark 

 of an induction coil passes, immediately charging the 

 Leyden-jar c, which discharges across the gap at e an 

 instant later. The capacity of the jar is so regulated 

 that the interval between the two sparks is about one 



