Description1911 Britannica - Astronomy - Telescope.png
English: Practical Astronomy. Let OF be a section of the telescope, MN being its object glass. Let the parallel dotted lines represent rays of light emanating from the object to be observed, which, for our purpose, we regard as infinitely distant, a star for example. These rays come to a focus at a point F lying in the focal plane of the telescope. Illustration from 1911 Encyclopædia Britannica, article Astronomy.
This image comes from the 13th edition of the Encyclopædia Britannica or earlier. The copyrights for that book have expired in the United States because the book was first published in the US with the publication occurring before January 1, 1929. As such, this image is in the public domain in the United States.
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