Lens
A lens was a transparent disc through which one could look. It was used to enhance or alter vision.
In 2286, James T. Kirk still owned the antique glasses that were given to him as a gift from Dr. Leonard McCoy; however, the lenses were cracked due to a battle he had previously had with Khan. He ended up selling the glasses to Feinberg's Loan and Pawn when he and the rest of his crew time traveled back to 1986 San Francisco with the HMS Bounty. (Star Trek IV: The Voyage Home)