PHOTOGRAPHY OF SOUND WAVES. 



361 



densit.v than the surrounding air {h). 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 tield; consequentlj- 

 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 raj^s 

 which traverse the luider part of ''5," 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 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 por- 

 tion of the medium in front of the lens will be sufficient to raise or 

 depress the iniag<^ 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 appar- 

 ent, since variations in the densit}" of the glass of the lens will act in 

 the same w^ay as variations in the density of the medium before it and 



Fl<i. 2. 



produce unequal illumination of the field. It is impossible to find a 

 lens which will give an absolutely 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 den- 

 sity, 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 ])y starting the sound wave 

 with an electric spark and illuminating it with the flash of a second 

 spark occurring a moment later, while the Avave was still in tiie 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 r. whicli discharges 

 across the gap at e an instant later. The capacity of the jar is so 



