PHOTOGKAPHY OF SOUND WAVES. 365 



(•lisped, and those eiisps trace the caustic emeloped ])y the twioe-reliected 

 ravs. These foriiLS can also be constructed geometrically. 



A much more complicated case is now shown (%. 8. PI. 11). Here the 

 wave starts within a complete sphere or rather cylinder. (Cylindrical 

 surfaces have been used in all these cases for obvious reasons, the sec- 

 tional views shown in the photographs being the same for both forms 

 of surface.) Starting in the principal focus of the closed mirror, the 

 wave is bounced back and forth, becoming more complicated after each 

 reflection, yet always symmetrical about the axis. Onlv a few of the 

 many forms are shown, and with the exception of the first three or 

 four are not arranged in order: for at the time that the series were 

 arranged, on the slide this case had not been worked out geometricallv, 

 and it was (|uite impossible to determine the evolution of the diflerent 

 forms. ]More recentl}- this case has been constructed for Hao reflec- 

 tions and all of the forms shown in the photographs found. 



We will take up next some cases of refraction, the first being that of 

 a spherical wave at a flat surface of a denser mediimi. In fig. !•. PI. Ill, 

 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 b}' Toepler. Ordinary collodion is diluted with 

 about ten parts of ether, poured on a small piece of plate glass, and 

 immediately drained ofl'. 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 better success by proceeding in the fol- 

 lowing manner: One end of the plate is lowered into a shallow dish of 

 water and the plate inclined until the water comes up to one of the 

 cuts. Bv looking at the reflection of a window in the water it is pos- 

 sible to see whether the film commences to detach itself from the glass. 

 If all goes well, it will float otf on the surface of the water along the 

 line of the knife cut, and it should be slowly lowered (one end resting 

 on the bottom of the dish) until the rectangular piece detaches itself and 

 floats freely on the surface. The edges of the tank are well greased, 

 and then lowered carefully upon the film, to which they will adhere. 

 The whole must then be lifted from the water in an obliciue direction, 

 when the film will be found covering the tank and exhibiting the most 

 beautiful interference colors. The tank was filled with carbonic acid 

 and placed under the origin of the sound wave. On striking the col- 

 lodion film the v.-ave is partly reflected and partly transmitted, and it 

 will be seen that the reflected component in air has moved farther than 

 the transmitted component in the carbonic acid. The spherical wave 

 front is transformed into an hyperboloid on Altering the denser 

 medium. This is well shown in No. 8 of the series. In No. 4 the 

 wave is seen in air, having been reflected up from the bottom of the 

 tank. 



SM 1900 26 



