Glass, when stressed, shows the property of double refraction: light of differing polarization directions bends differently passing through the glass. When placed between crossed polarizers , bands of color may be seen, with the spacing of the bands being correlated to the amount of stress present at a particular point: small spacing indicates large stress. These devices were for stressing glass samples to demonstrate this effect.
Property of the W&L Physics & Engineering Department
“Glass Presses,” Digital Exhibits at Washington and Lee University Library, accessed May 26, 2018, https://omeka.wlu.edu/items/show/179.