APERTURE, in anatomy, zoology, botany, etc.: (a) The aperture of a univalve shell is the opening or mouth. In mollusks which feed on vegetable matter it is entire; while in those which are animal feeders it has a notch or canal. In some families it has an oper culum or cover. The margin of the aperture is called the peristome. (b) Any other opening.
In optics, the diameter of the object glass of a refracting telescope, or the speculum or mirror of a reflector. The larger the aperture (i. e., the area of the surface through which the light is trans mitted, or from which it is reflected), the greater is the power of the telescope to penetrate into space and consequently bear higher magnifying powers. The great refractor at the United States Ob servatory at Washington has an aperture of 26 inches. In recent years silver glass parabolic mirrors of the New tonian form have been constructed with large apertures and short focal length, thus rendering these instruments exceedingly convenient for use. Sir W.
Herschel's 18-inch metallic speculum, used for examining the nebula; and Milky Way, had a focal length of 20 feet; modern telescopes, with silvered-glass mirrors, have been constructed of the same aperture, but with a focal length of not more than seven feet.
Angular aperture (in microscopes), the amount of light transmitted by the objective, and, consequently, the distinct ness of the image afterward magnified by the lenses forming the eye piece. When an objective of the large angular .perture is employed, the more delicate .narkings of the object under examina tion, invisible when objectives of less angular aperture are used, are seen with great distinctness.