"watch glass." YourDictionary, n.d. Web. 10 November 2018. <https://www.yourdictionary.com/watch-glass>.
watch glass. (n.d.). Retrieved November 10th, 2018, from https://www.yourdictionary.com/watch-glass
(plural watch glasses)
(chemistry) An item of glass laboratory equipment consisting of a simple slightly concave circle of glass. It is a general utility item, used to hold small samples, to evaporate liquids, to cover a beaker, and so on.
English Wiktionary. Available under CC-BY-SA license.