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Object-Glass is that part of a microscope or telescope which forms the first image of the object viewed. When a single lens is used as a magnifying-glass it may be said to combine the functions of both object-glass and eye-piece, and in this case the image formed is erect and virtual. In the case of microscopes and telescopes, however, the object-glass forms an inverted real image in the instrument, this image being viewed by the eye-piece. In all good instruments the object-glass is an achromatic combination of lenses, generally of flint and crown glass.

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