E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Spectacles, or rather reading-glasses; so called because in shape they resemble the twitchers used by farriers to keep under restraint unruly horses during the process of bleeding, dressing, or shoeing. This instrument, formerly called a barnacle, consisting of two branches joined at one end by a hinge, was fixed on the horses nose. Dr. Latham considers the word a corruption of binocles (double-eyes), Latin, binus oculus. Another suggestion is binnacle, the case on board ship in which the steering compass is placed, illuminated when it is dark by a lamp.