August 9, 1900] 



NATURE 



345 



worked out geometrically, and it was quite impossible 

 to determine the evolution of the different forms. More 

 recently this case has been constructed for five reflec- 

 tions, and all of the forms shown in the photographs 

 found. 



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







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 re- 

 flected 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. In 

 No. 4 the wave is seen in air, having been re- 

 flected up from the bottom of the tank. 



being that of a spherical wave at a flat surface of a 

 denser medium. In Fig. 9 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 immediately drained ofif. 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 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 com- 

 mences to detach itself from the glass. If all goes well, 

 it will float ofTon the surface of the water along the line 

 of the knife-cut, and it should be slowly lowered (one 



Fig. 9. 



end resting on the bottom of the dish) unti the rect- 

 angular 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 oblique direction, when the film will be found 



NO. 1606, VOL. 62I 



In Fig. 10 we have the refraction of the wave 

 in the same tank under oblique incidence. The 

 bending of the wave within the tank is very 

 marked. The wave-fronts reflected from the 

 side which follows the unreflected portion 

 is also interesting in connection with Lloyd's 

 single mirror interference experiment (No. 2 of 

 series). 

 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. 1 1 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. : 



