APERTURE, in optics, has been defined in the Encyclopædia, but no rule was given there for finding a just aperture. As much depends upon this circumstance, our optical readers will be pleased with the following practical rule given by Dr Hutton in his Mathematical Dictionary. "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 distant object, as the moon, stars, &c. then that aperture is to be chosen through which they appear the most distinctly."
"Fluygens first found the use of apertures to conduce much to the perfection of telescopes; and he found by experience (Diops. prop. 56.), that the best aperture for an object-glass, for example of 30 feet, is to be determined 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 proportioned to the apertures. And M. Auzout says he found, by experience, that the apertures of telescopes ought to be nearly in the sub-duplicate 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
Apecatastasis 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 shown by Dr Hook, Phil. Trans. No 4.