APERTURE
, in Geometry, is used for the space left between two lines which mutually incline towards each other, to form an angle.
Aperture, in Optics, is the hole next the objectglass of a telescope or microscope, through which the light and the image of the object come into the tube, and are thence conveyed to the eye.
Aperture is also understood of that part of the object-glass itself which covers the former, and which is left pervious to the rays.
A great deal depends upon having a just aperture.— To find it experimentally: apply several circles of dark paper, of various sizes, upon the face of the glass, from the breadth of a straw, to such as leave only a small hole in the glass; and with each of these, separately, view some distinct objects, as the moon, stars, &c; then that aperture is to be chosen through which they appear the most distinctly.
Huygens first found the use of apertures to conduce much to the perfection of telescopes; and he found by experience (Dioptr. prop. 56.) that the best aperture for an object-glass, for example of 30 feet, is to be de termined by this proportion, as 30 to 3, so is the square root of 30 times the distance of the focus of any lens, to its proper aperture: and that the focal distances of the eye-glasses are proportional to the apertures. And M. Auzout says he found, by experience, that the apertures of telescopes ought to be nearly in the subduplicate ratio of their lengths. It has also been found by experience that object-glasses will admit of greater apertures, if the tubes be blacked within side, and their passage furnished with wooden rings.
It is to be noted, that the greater or less aperture of an object-glass, does not increase or diminish the visible area of the object; all that is effected by this, is the admittance of more or fewer rays, and consequently the more or less bright the appearance of the object. But the largeness of the aperture or focal distance, causes the irregularity of its refractions. Hence, in viewing Venus through a telescope, a much less aperture is to be used than for the moon, or Jupiter, or Saturn, because her light is so bright and glaring. And this circumstance somewhat invalidates and disturbs Azout's proportion, as is shewn by Dr. Hook, Philos. Trans. No. 4.
