362 PHOTOGRAPHY OF SOUND WAVES. 



regulated that the interval between the two sparks is about one ten- 

 thousandth of u second. The field of the lens is thus illuminated I)}' 

 the flash of the second spark before the sound wave started by the first 

 spark has gone beyond the edge of the lens. 



To secure the proper time interval l)etween the two sparks it is nec- 

 essary that the capacity of the jar l)e quite small. A good-sized test 

 tube half full of mercury standing in a jar of mercury is th(> easiest 

 arrangement to fit up. This limits the length and brilliancy of the 

 illuminating spark, and with the device employed l)y Toepler 1 was 

 unable to get enough light to secure photographs of the waves. 

 After some experimenting T found that if the spark of the jar was 

 passed between two thin pieces of magnesium ribbon pressed between 

 two pieces of thick plate glass a very marked improvement resulted. 

 With this form of illuminator T found that five or six times as much 

 light could !»<' obtained as by the old method of passing the spark 

 between two hi-ass balls. 



The spark is flattened out into a band and is ke])t always in the 

 same plane, the light issuing in a thin sheet from between the plates. 

 By this arrangement we secure a light source of considerable length, 

 great intensity, and bounded by straight edges, th(» three essentials 

 for securing good results. The glass plates, with the i-ibbon termi- 

 nals ])etween them, nuist be clamped in some sort of a holder and 

 directed so that the thin sheet of light strikes the lens. This can be 

 accomplished by darkening the room, fastening a sheet of paper in 

 front of the lens, and then adjusting the plates so that the paper is 

 illuminated as much as possible. The image formed by the lens will 

 be found to have very sharp straight edges.' on one of which the 

 edge or the diaphragm can be set in such a manner as to allow but 

 very little light to pass when the intervening medium is homogeneous. 

 A very slight change, however, in any portion ma}' be sufficient to 

 cause the entire amount of light passing through that portion to pass 

 below the diaphragm and enter the telescope. 



The photographs were made by substituting a photographic objec- 

 tive for the telescope in the focal plane of which a vertical board was 

 mounted to support the plate. The room was darkened, a plate held 

 in position, and a single spark made to pass between the knobs by 

 pulling a string connected with the hammer of the induction coil. 

 The plate was then moved a trifle and a second impression secured in 

 the same way. This obviated several of the difticulties experienced 

 in the earlier work. The images never overlapped, and the hot air 

 from the spark did not appear in the pictures. About thirty -five 

 images were obtained on each plate in less than a minute, from which 



' If more than one image appears it means that the plane of the glass plates of 

 the illmninator does not lie parallel to the optical axis of the system. It is of prime 

 importance to secure a single image. 



