Note:  Do not rely on this information. It is very old.

Opera Glass

Opera-Glass (Galileo's telescope) is a form of telescope, but, as its name implies, is chiefly used to render objects or persons on the stage of a theatre more distinct. It consists of an achromatic object glass, which would naturally form a real inverted image of the object. The rays are, however, intercepted by the eye-piece before they can form that image, and the eyepiece being a concave achromatic lens forms a magnified erect image of the object which the observer sees. The special utility of the instrument is that it gives at once an erect image without any contrivance for reinversion, and its length is so small that it is easy to hold and control.