When we refer to an instrument of vision, what we really mean is something that helps us see better. While the term itself is perfectly fine, sometimes it's useful to have synonyms available to help us express ourselves more precisely. Some potential alternatives include "optical device," "ocular apparatus," "visual aid," "eyepiece," or even simply "spectacles." Depending on the context and the level of formality, there may be other options available as well. By having a range of vocabulary to draw upon, we can communicate our thoughts with greater clarity and accuracy.