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THE WONDERS OF OPTICS.
observation than that given by the single bi-concave. This combination is commonly known as the astronomical eye-piece. It reverses the object looked at, but for astronomical purposes this defect is of no consequence.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/6/66/The_wonders_of_optics_-_Fig_42.png/400px-The_wonders_of_optics_-_Fig_42.png)
Fig. 42.—The Astronomical Telescope.
The instrument shown in the above figure represents an astronomical telescope reduced to its simplest form.
Fixed parallel to the axis of the larger telescope is