Sound-Waves by the " Schlieren-Methode." 223 



The wave now begins to diverge again, and moves up out 

 of the reflector, a portion encountering the walls and suffering 

 reflexion. These reflected portions appear to be converging 

 shells, for they are always " concave up " until they get to 

 the top of the mirror, when they are " concave down." In 

 one or two pictures they seem to have come to a focus, 

 although the point is not as sharply defined as the principal 

 focus. It will be seen tbat light and shade of the wave are 

 interchanged after the passage of the disturbance through 

 the focus. This I am wholly unable to explain. It must be 

 remembered that the light and shade of the wave do not 

 correspond to rarefaction and condensation, but depend merely 

 on the position of the diaphragm which cuts off a portion of 

 the image of the spark. It will be seen that the waves are 

 always light above and dark below, or vice versa, according 

 as the diaphragm is above or below the image. If the edge 

 of the diaphragm covers the upper edge of the spark's image, 

 any portion of the medium in front of the lens which has an 

 optical density greater than the surrounding medium (its 

 density, however, shading off gradually to that of the rest of 

 the medium) will appear bright above and dark below, since 

 the rays which traverse the upper portion are bent down and 

 get under the diaphragm's edge, increasing the illumination, 

 while the rays traversing the under portion are bent up, and 

 are consequently intercepted by the diaphragm. A region 

 of less optical density would act in the opposite manner, and 

 appear dark above and light below. 



Some of the photographs were taken with the diaphragm 

 above the spark, and some with it below, consequently both 

 arrangements of light and shadow will be found in the 

 pictures. 



The reflexion of a wave does not affect the distribution of 

 the light and shade, but passage through a focus always 

 seems to, the appearance being what we shculd expect if a 

 condensation-wave swept into the focus and a rarefaction-wave 

 swept out, a circumstance that is scarcely possible. 



The refraction of sound in a medium denser than air is 

 shown in fig. 5, where we have a rectangular tank with sides 

 made of plane-parallel glass, and covered with a collodion film 

 of soap-bubble thickness made by the method described by 

 Toepler. Ordinary collodion is diluted with about ten parts 

 of ether, poured on a small piece of plate-glass and im- 

 mediately drained off. As soon as it is quite dry a rectangle 

 is cut with a sharp knife on the film. Toepler's method of 

 removing the film was to place a drop of water on one of the 

 cuts, and allow it to run in by capillarity ; but I have had 



