In such a case, no tube could be used, and the object glass was merely fixed at the top of a high pole.

To these achromatic object glasses, as they are called, the great development of astronomical knowledge, since Newton's time, is due.

No doubt if this had been the case, a perfect telescope could have been produced by properly shaping the object glass.

It is usually a double-image micrometer used to measure the diameter of the image of the object glass.

Objectives are also called object lenses, object glasses, or objective glasses.

The measurement given directly is that of the image of the object formed at the focus of the object glass.

One of the most difficult operations of practical optics was to polish the spherical surfaces of large object glasses accurately.

The eye pupil should coincide with the Ramsden disc, the image of the entrance pupil, which in the case of an astronomical telescope corresponds to the object glass.

In certain forms of lenses there are two such foci; and it is by taking advantage of this fact that the best aplanatic object glasses of microscopes are constructed.

In the thicker and larger glasses, there would be more of such defects, so that in larger telescopes this kind of glass would not be fit for object glasses.

The reason was mundane enough - the binnacle drawer had opened and slid out a couple of days ago and both telescopes in it had landed on the deck and cracked their object glasses.

On these is firmly attached a small telescope, furnished with cross wires, or, what is better, crossed portions of the fine balance-spring of a watch, set flat-ways, and adjusted very exactly in the sidereal focus of object glass.

The telescope is focused correctly for viewing objects at the distance for which the angular magnification is to be determined and then the object glass is used as an object the image of which is known as the exit pupil.

It had been supposed that the reason why success had not been attained in the construction of a refracting telescope was due to the fact that the object glass, made as it then was of a single piece, had not been properly shaped.

He presented the Royal Greenwich Observatory with several instruments, including a photographic heliograph of 9-inch aperture; a 30-inch reflecting telescope, and a large refracting telescope having an object glass of 26 inches of diameter and a focal length of 22 feet.

A peal of six bells struck out, human faces began to crowd the windows around, and the procession of heads of houses and new doctors emerged, their red and black gowned forms passing across the field of Jude's vision like inaccessible planets across an object glass.

This apparatus, provided with a powerful object-glass, was very complete.

He lowered the telescope, wiped the object-glass, and peered again.

Note that the function of the object-glass is to collect the light, and all the actual magnification is done by the eyepiece.

Here, there is no object-glass.

The first, constructed by Herschel, was thirty-six feet in length, and had an object-glass of four feet six inches; it possessed a magnifying power of 6,000.

The first application of the divided object-glass and the employment of double images in astronomical measures is due to Servington Savary from Exeter in 1743.

However, Airy wrote the first full theoretical treatment explaining the phenomenon (his 1835 "On the Diffraction of an Object-glass with Circular Aperture").

The clock consisted of a spyglass, having a nicol (double-image) prism for an eyepiece, and a thin plate of selenite for an object-glass.

'There is a ship on fire,' said Jack in horror, his heart pumping so hard that he could scarcely keep the steady deep-red glow in his object-glass. '

John Dollond, in 1754, combined Savary's idea of the divided object-glass with Bouguer's method of measurement, resulting in the construction of the first really practical heliometers.

A piece of flint-glass by Guinand, nearly seven inches across, purchased by him in 1823 for £250, was worked by Tulley into the largest object-glass then in England.

According to the calculations of the Observatory of Cambridge, the tube of the new reflector would require to be 280 feet in length, and the object-glass sixteen feet in diameter.

If the object-glass of a refractor or the mirror of a reflector is of poor quality, the images will also be poor - and a bad telescope does not always betray itself at a glance.

With the refractor, the light from the object to be observed is collected by an object-glass; the light is brought to focus, and the image is then enlarged by a second lens termed the eyepiece.

As far as we can ascertain, Joseph von Fraunhofer, some time not long before 1820, constructed the first heliometer with an achromatic divided object-glass, i.e. the first heliometer of the modern type.

Instruments:--1 sextant, 1 double opera-glass, 1 telescope, 1 box of mathematical instruments, 1 mariner's compass, 1 Fahrenheit thermometer, 1 aneroid barometer, 1 box containing a photographic apparatus, object-glass, plates, chemicals, etc.

His first undertaking at the Dublin Observatory was the erection of an equatorial telescope to carry the fine object-glass presented to the university by Sir James South; and on its completion he began an important series of researches on stellar parallax.

Huygens's 123-foot object-glass, lent to Pound in 1717 by the Royal Society, was mounted by him in Wanstead Park on the maypole just removed from the Strand, and procured for the purpose by Sir Isaac Newton.

The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter, son of, John Dollond, and many excellent telescopes of this kind were made by him.

In the same year the Duke of Northumberland presented the Cambridge observatory with a fine object-glass of 12-inch aperture, which was mounted according to Airy's designs and under his superintendence, although construction was not completed until after he moved to Greenwich in 1835.

At one end there is the object-glass, and at the other end the eye-piece, and of course it is obvious that with an instrument of this construction it is to the lower end of the tube that the eye of the observer must be placed when the telescope is pointed to the skies.