“Great men,” wrote Ralph Waldo Emerson, “are a collyrium to clear our eyes from egotism, and enable us to see other people and their works.” At the time, 150 years ago, his educated readers would have been familiar with the word in the sense of a medicated salve for the eyes (and with its plural collyria), though it’s now much less common.
In fact, it’s all Greek to most of us, which is only reasonable since it’s originally Greek, from kollurion, a poultice, a word taken from kollura, a type of coarse bread roll.
The Greek writer Lucian used it in the first century BC to explain how a trickster was able to remove seals from documents and replace them undetected afterwards. One method was to make a mould of the seal using “the substance called collyrium; this is a preparation of Bruttian pitch, bitumen, pounded glass, wax, and mastic”. Not the sort of thing you’d want near your eyes.
The shift in sense came about in part because the Romans and their successors gave the name to a variety of solid medicines that were made up into cakes held together by gum; these were dissolved in some suitable liquid before being applied to the body, especially the eyes. It’s also a name given to a dark eye shadow or kohl used in some Eastern countries.