<a href="https://en.wikipedia.org/wiki/Lissajous_curves" class="extiw" title="en:Lissajous curves">Lissajous curves</a> are created when a beam of light bounces off mirrors attached to a pair of vibrating tuning forks. From <i>Physics in Pictures. The principal natural phenomena and appliances described and illustrated by thirty coloured plates, for ocular instruction in schools and families</i>, plate XXII.
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