OPERA GLASS. A small double telescope. used for looking at objects that require to be seen clearly and distinctly rather than greatly magnified. The opera glass is short and light. and though it has usually small magnifying power (varying in most instances from two to three times), the large amount of light admitted by the object glass on account of its large angle of apertIu•e enables it to present a well-illumi nated picture which can be seen without undue strain to the eye. The opera glass allows the use of both eyes. which gives to the spectator the advantage of seeing objects stand out stereo scopically as in ordinary vision. It consists of two lenses, or sets of lenses, as each lens is gen erally achromatic and made up of two lenses of different glass fitted together. The object lens, which is the larger, is convex, and the eye lens is concave. They are mounted so that when the tubes are drawn out the distance between the two lenses shall be nearly equal to the difference of their focal lengths. The figure shows the
action of the opera glass. 0 is the object lens and E is the eye lens, the line __'.representing the axis of the instrument. The object lens alone would form a real and inverted image CB' of the distant object LM at or near its principal focus, were it not for the concave eye lens which /divulges the direction of the rays and causes them to diverge instead of converge at the focus.
Accordingly, the rays appear to diverge from an erect and magnified image located at BC. The formation of the image may he understood by tracing the course of rays, r, r, r, diverging from some point Of the original object such as L. These rays diverge until they meet the convex lens 0, which makes them convergent, and would bring them to a focus at B', but the concave lens E causes them to diverge and take the direction of the rays r', r', entering the eye as if they came from the point B.