Visibility of David Crighton and ultrasonic waves

Visibility of David Crighton is assured through his many useful publications. He was able to make his theoretical results seem simple and understandable. Visibility of ultrasonic waves is another matter. In our understanding we are aided by another Fellow of the Royal Society. Lord Rayleigh used collimated light incident on a grating. He showed that behind the grating there is a series of real images with an optical periodicity of magnitude (2b2)/l where b is the grating spacing and l is the optical wavelength. For the ultrasonic analog we substitute a standing ultrasonic wave for the grating, then we superimpose a scale on the images of the ultrasonic wave‐fronts. This becomes a simple but sensitive means of measuring ultrasonic wave velocity in transparent media.