Sound-Waves by the "Schlieren-Methode." 221 



that five or six times as much light could be obtained as by 

 the old method of passing the spark bet-ween two brass balls. 



The spark is flattened out into a band, and is kept always 

 in the same plane, the light issuing in a thin sheet from 

 between the p'ates. By this arrangement we secure a light 

 source of considerable length, great intensity, and bounded 

 by straight edges, the three essentials for securing good 

 results. The glass plates, with the ribbon terminals between 

 them, must be clamped in some sort of a holder and directed 

 so that the thin sheet of light strikes the lens : this can be ac- 

 complished 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 of 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 may be sufficient to cause 

 the entire amount of light passing through that portion to 

 pass below the diaphragm and enter the telescope. 



This arrangement of the spark-terminals between plates of 

 glass is also convenient for spectroscopic work, as the spark 

 is concentrated into a very narrow bright line, which is fixed 

 in position instead of wandering about, as it is apt to do when 

 passed between uncovered terminals. 



For photographing the waves the telescope was removed, 

 and a Zeiss photographic objective put in its place. A vertical 

 board was firmly clamped behind this in such a position that 

 the image of the balls, between whicii the sound-spark passed, 

 would be in focus on a plate held against it. This arrange- 

 ment was used instead of a camera, because it was necessary 

 to move the plate rapidly during the exposure, to prevent the 

 image of more than one wave being formed on the same 

 place. It was found that simply holding the plate in the 

 hand against the vertical board and advancing it slowly from 

 left to right, at the same time giving it a rapid up-and-down 

 motion, answered every purpose. It must be borne in mind 

 that the image of the 5-inch lens on the plate was only about 

 13 mm. in diameter, and that the field surrounding it was 

 dark ; consequently a large number of images could be 

 secured on a single plate. 



These images showed the waves in different stages of 

 development, for the time-interval between the two sparks 

 varied between rather wide limits. This was really an 

 advantage, for on a single plate it was possible to pick out a 

 series showing the successive changes in the form of the 



