outer surface. This stress distribution is measured by using glass's natural property of stress-induced birefringence and by employing techniques of 3D photoelasticity. The high fracture toughness due to residual compressive stresses makes Prince Rupert's drops one of the earliest examples of toughened glass. History It has been suggested that methods for making the drops have been known to glassmakers since the times of the Roman Empire. Sometimes attributed to Dutch inventor Cornelis Drebbel, the drops were often referred to as lacrymae Borussicae (Prussian tears) or lacrymae Batavicae (Dutch tears) in contemporary accounts. Verifiable accounts of the drops from Mecklenburg in North Germany appear as early as 1625. The secret of how to make them remained in the Mecklenburg area for some time, although the drops were disseminated across Europe from there, for sale as toys or curiosities. The Dutch scientist Constantijn Huygens asked Margaret Cavendish, Duchess of Newcastle to investigate the properties of the drops; her opinion after carrying out experiments was that a small amount of volatile liquid was trapped inside. Although Prince Rupert did not discover the drops, he played a role in their history by bringing them to Britain in 1660. He gave them to King Charles II, who in turn delivered them in 1661 to the Royal Society (which had been created the previous year) for scientific study. Several early publications from the Royal Society give accounts of the drops and describe experiments performed. Among these publications was Micrographia of 1665 by Robert Hooke, who later would discover Hooke's Law. His publication laid out correctly most of what can be said about Prince Rupert's drops without a fuller understanding than existed at the time, of elasticity (to which Hooke himself later contributed) and of the failure of brittle materials from the propagation of cracks. A fuller understanding of crack propagation had to wait until the work of A. A. Griffith in 1920. In 1994, Srinivasan Chandrasekar, an engineering professor at Purdue University, and Munawar Chaudhri, head of the materials group at the University of Cambridge, used high-speed framing photography to observe the drop-shattering process and concluded that while the surface of the drops experiences highly compressive stresses, the inside experiences high tension forces, creating a state of unequal equilibrium which can easily be disturbed by breaking the tail. However, this left the question of how the stresses are distributed throughout a Prince Rupert's drop. In a