The context is an article about Sir Christopher Zeeman, the mathematician who founded the Maths Institute at the University of Warwick (UK) back in the 50s or 60s. Talking about how they tried to emphasise the social side of things at this new Institute, the article reads:

"Morning coffee and afternoon tea were served from elegant china in a comfortable common room where staff, graduates and visitors gathered for mathematical conversations. Greenglasses were available around the walls so that diagrams could be drawn at the drop of a saucer."

What are these greenglasses? I can't find the term in any of my dictionaries, and I'm curious to know.