Photography of Ripples. 417 



vibrations. The light curved lines in the region between the 

 source and the nearest side of the triangle are similar to 

 those between the two sources in fig. 2. They are due to 

 the interference between the source and its virtual image. 



Fig. 8. Frequency 256. 



Here we have a shallow circular reflector with the source 

 placed approximately at the principal focus. The reflected 

 waves are circles of large radii ; the very slight outward cur- 

 vature at the ends shows that even when the reflector has 

 an arc of about 60° the effect of spherical aberration is small. 

 Since the reflected waves come from a virtual point-source, 

 we have, as interference-lines, a series of confoc.il ellipses and 

 confocal hyperbolas ; the latter are fixed, but the former travel 

 away from the line joining the source and its image. If the 

 reflected waves had been rectilinear, both these sets of curves 

 would have become parabolas. 



Fig. 9. Frequency 256. 



The centre of disturbance here coincides very closely with 

 the principal focus of the central portion of the semicircular 

 reflector. The reflected ripples are straight lines in the 

 middle, but are bent outwards from the reflector towards the 

 ends. This illustrates spherical aberration. 



Fig. 10. Frequency 256. 



The obstacle is a small round cover-slip floating on the 

 mercury. The ripple-shadow is slightly encroached upon by 

 the waves bending round the edge of the obstacle. One side 

 of the disk acts as a convex circular mirror, and the inter- 

 ference-fringes are due to the mutual action of the source 

 and its virtual image situated within the circumference of the 

 disk. 



Fig. 11. The frequency in this and fig. 12 is 120. 



Here straight-line waves are originated by the agitation of 

 a slip of cover-glass, one side of which dips into the mercury. 

 The waves are reflected from the shallow circular mirror, and 

 converge to the principal focus. Two series of parabolic 

 interference-fringes are shown. They are confocal, and have 

 their concavities directed towards the source and reflector 

 respectively. 



Fig. 12. Similarly excited waves are reflected at an angle 

 of about 45° from a straight edge. The long black mark 

 running from one end of the dipping edge to the corner of 

 the print is due to a depression in the surface caused by a 

 floating needle, put there to screen off the circular waves 

 coming from the end of the strip of glass. 



Diffraction is well shown in this photograph. 



