592 



On the Propagation of Cusped Waves. 



shown in cross section in fig. 2, is extremely useful in making 

 the whole matter clear. It can be made by drawing down a 

 large thin tube ; melting the end down flat, and then sucking 

 it in a little. 



Fig. 2. 



Axis 



uv 



m- r1 



Another useful piece of apparatus can be made by silvering 

 the outside of a hemispherical glass evaporating-dish or half 

 of a large round-bottomed flask. The concave mirror thus 

 formed should be mounted on a stand, and a two-candle 



i : / 



^ \ 



power " pea " electric lamp arranged so that it can be moved 

 along the axis of the mirror. In my second paper on the 

 photography of sound-waves it was shown that a spherical 



