further study published in 2017, the team collaborated with Hillar Aben, a professor at Tallinn University of Technology in Estonia using a transmission polariscope to measure the optical retardation of light from a red LED as it travelled through the glass drop, and used the data to construct the stress distribution throughout the drop. This showed that the heads of the drops have a much higher surface compressive stress than previously thought at up to , but that this surface compressive layer is also thin, only about 10% of the diameter of the head of a drop. This gives the surface a high fracture strength which means that it is necessary to create a crack that enters the interior tension zone in order to break the droplet. As cracks on the surface tend to grow parallel to the surface, they cannot enter the tension zone but a disturbance in the tail allows cracks to enter the tension zone. A scholarly account of the early history of Prince Rupert's drops is given in the Notes and Records of the Royal Society of London, where much of the early scientific study of the drops was performed. Scientific uses The process for the production of toughened glass by quenching was probably inspired by the study of the drops, as it was patented in England by Parisian Francois Barthelemy Alfred Royer de la Bastie, in 1874, just one year after V. De Luynes had published accounts of his experiments with them. It has been known since at least the 19th century that formations similar to Prince Rupert's drops are produced under certain conditions in volcanic lava. More recently researchers at the University of Bristol and the University of Iceland have studied the glass particles produced by explosive fragmentation of Prince Rupert's drops in the laboratory to better understand magma fragmentation and ash formation driven by stored thermal stresses in active volcanoes. Literary references Because of their use as a party piece, Prince Rupert's drops became widely known in the late 17th century—far more than today. It can be seen that educated people (or those in "society") were expected to be familiar with them, from their use in the literature of the day. Samuel Butler used them as a metaphor in his poem Hudibras in 1663, and Pepys refers to them in his diary. The drops were immortalized in a verse of the anonymous Ballad