224 Mr. R. W. Wood on Photography of 



better success by proceeding in the following 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. By looking at the reflexion of a window in the water 

 it is possible to see whether the film commences to detach 

 itself from the glass. If all goes well it will float off 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. Ttie 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 

 oblique direction, when the film will be found covering the 

 tank and exhibiting the most beautiful interference-colours. 

 The tank was filled with carbonic acid and placed under the 

 origin of the sound-wave. On striking the collodion-film the 

 wave 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 

 entering the denser medium. This is well shown in No. 3 of 

 the series, where the wave in air, moving at higher velocity, 

 has passed out of the field entirely, and there remains only the 

 slower-moving disturbance iu the denser gas. In No. 4 the 

 wave is seen returning up through the tank after having 

 struck the bottom. 



After several failures I succeeded in constructing a prism 

 with its two refracting faces of this exceedingly thin collodion, 

 which, when filled with carbonic acid, showed the bending of 

 the wave-front, exactly as we figure it in diagrams for light. 

 It was necessary to have the collodion thinner than before, 

 since if we are to photograph the wave after twice traversing 

 the film, we must lose as little energy as possible by reflexion. 

 Fig. 6 shows the refraction in a carbonic-acid prism, the 

 bending being particularly noticeable in No. 4, on which I have, 

 with a pair of dividers, traced out the position which the wave- 

 front would have occupied had it not traversed the prism. 



Fig. 7 shows the refraction of the wave when the tank was 

 filled with hydrogen, in which the sound travels faster than 

 in air. The bulging down of the wave-front is very notice- 

 able, though it is not as great as we should expect. This 

 may be due to the fact that the hydrogen is not pure, owing 

 to diffusion, but as a steady stream was flowing from a 

 generator into the tank throughout the experiment, and as 

 no " Schlieren " are to be seen, I do not think that this can 

 fully explain the discrepancy. 



