708 Mr. W. Bennett on 
wave-front first formed. The result is seen in fig. 2, which 
gives a better view of the form of the waves in this region 
Fig. 2 

than can be obtained from the smaller figure given by Prof. 
Wood. The waves are shown as proceeding from a mirror 
of aperture 52° lying to the left of the diagram ; the complete 
forms are given by Prof. Wood in the paper already referred 
to. Each wave-front is a figure of revolution consisting of a 
saucer-shaped part in front, bounded by a circular cuspidal 
edge which is tracing out the caustic surface, and a trailing 
part behind, which has already passed the caustic. 
The wave-fronts are given by the equations 
r= { K—a(cos + 7) \ sin 20, 
a 
e=9" COL 2D ig ae 
wnere K is a parameter constant over any one wave-front, 
being, in fact, the optical distance from the incident wave- 
front through the centre of curvature. These equations, 
however, are not easily worked with. 
Imagine the wire placed so as to meet only the trailing 
part of the wave-front ; as the curvature of the wave-front 
is not uniform, the trace left upon it by the wire will be 
distorted as the wave advances, and the shadow will be a 
single-branched curve on the same side of and farther from 
the axis. This curve will be concave to the axis in its central 
parts, and will have two points of inflexion and an asymptote 
parallel to the wire. 
