APERTURE

APERTURE, Apertura, the Opening of anything; or a Hole, Cleft, or vacant Place in some otherwise solid, or continuous Subject. See OPENING.
The Word comes from the Latin Apertura, of Aperire to open.
In Geometry, Aperture is used for the Space left between two Lines which mutually incline towards each other to form an Angle. See ANGLE.
In Optics, Aperture is the Hole next the Object-Glass of a Telescope, or Microscope; through which the Light and Image of the Object come into the Tube, and are thence carried to the Eye. See OBJECT-GLASS.
Aperture is also understood of the Part of the Object Glass itself, which covers the former, and which is left pervious to the Rays. See TELESCOPE.
A great deal depends on having a just Aperture. To find it experimentally; apply several Circles of black smutted Paper, each bigger than other, upon the Face of the Glass, from the Magnitude of a Pea to the whole Extent of the Glass; and with each of these, separately, view several distant Objects; as the Moon, Stars, etc. That through which they appear the most distinctly, is to be pitched upon. M. Auzout affirms, that he found that the Apertures of Telescopes ought to be nearly in the subduplicate Ratio of their Lengths: But Huygens, who first introduced the Use of Apertures, assures us he found by experience, that the Aperture of an Object-Glass, e.g., of 30 Foot, is to be determined by this Proportion: as 30 to 3, that is, as 10 to 1, so is the Root of the Distance of the Focus of any Glass multiplied by 30, to its Aperture: and the focal Distances of the Eye-Glasses are to be proportional to the Apertures. A Table of Apertures for Telescopes of various Lengths, etc.
See under the Article TELESCOPE.
The greater or less Aperture of an Object-Glass, it is to be noted, does not increase or diminish the visible Area of the Object; all that is effected by this, is the admittance of more or less Rays, and consequently the more bright or obscure Appearance of the Object.



Hence, in viewing Venus through a Telescope, a much less Aperture is to be used than for the Moon, Jupiter, or Saturn, because her Light is so vivid and glaring. Which Consideration does a little invalidate and disturb M. Auzout’s Proportion, as is shown by Dr. Hooke, Phil. Transact.

Apertura Tabularum, in ancient Law-Books, signifies the breaking open a Last Will and Testament. See WILL and TESTAMENT.

Apertura Feudi, denotes the loss of a feudal Tenure, by default of issue to him, to whom the Feud or Fee was first granted. See FEE, FIEF, TENURE, etc.

APERTURE