This ring forms an optical illusion. The engraving makes the parallel lines on the ring appear to converge and diverge as you turn the ring on your finger. The optical effect is caused by the Zöllner illusion which was described by the German astrophysicist Johann Karl Friedrich Zöllner in 1860. An illustration of this type of ring was published by Gaston Tissandier in 'Les recréations scientifiques ou l'enseignement par les jeux' (1880), translated by Henry Frith as 'Scientific Amusements' (London, c. 1900). They were sold as 'magic pattern rings' by the London jewellers Garrard.
This kind of optical toy is a very old one. The French essayist Michel de Montaigne (1533-1592) described very similar rings in 'An Apology for Raymond Sebond':
"….those rings which are engraved with feathers of the kind described in heraldry as endless feathers – no eye can discern their width, or defend itself from the impression that from one side they appear to enlarge, and on the other to diminish, even when you turn the ring around your finger. Meanwhile if you measure them they appear to have constant width, without variation ….."
Inside the ring, a small locket fitting may once have held hair.