Dictionary.net

• An opera glass or field glass with an oblique mirror arranged for seeing objects do not lie directly before the eye; - called also diagonal, side, opera glass.

2010 - New Age Dictionary Database
By Oddity Software

• An opera glass or field glass with an oblique mirror arranged for seeing objects do not lie directly before the eye; - called also diagonal, side, opera glass.

1913 - Webster's Revised Unabridged Dictionary
By Noah Webster.