1911 Encyclopædia Britannica/Aperture

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Apenrade

1911 Encyclopædia Britannica, Volume 2

Aperture

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5772041911 Encyclopædia Britannica, Volume 2 — Aperture

APERTURE (from Lat. aperire, to open), an opening. In optics, it is that portion of the diameter of an object-glass or mirror through which light can pass free from obstruction. It is equal to the actual diameter of the cylinder of rays admitted by a telescope.