Pierre Bouguer, in 1748, originated the true conception of measurement by double image without the auxiliary aid of a filar micrometer, that is by changing the distance between two object-glasses of equal focus. 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. 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.
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Before long a mural circle was installed, and regular observations were instituted with it in 1833. 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. Airy's writings during this time are divided between mathematical physics and astronomy. The former are for the most part concerned with questions relating to the theory of light arising out of his professorial lectures, among which may be specially mentioned his paper On the Diffraction of an Object-Glass with Circular Aperture, and his enunciation of the complete theory of the rainbow.
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A dynameter is an instrument that measures the magnification of a telescope. It is usually a double-image micrometer used to measure the diameter of the image of the object glass. The magnifying power is found by comparing the actual diameter of the glass with the measured diameter of the image of the glass .
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Rutgers equipped the observatory with "a 6.5-inch equatorial refracting telescope, a meridian circle with four-inch object glass for transit observations, a sidereal clock, a mean solar clock...chronograph, repeating circle, and other instruments."Murray, David. Hand-Book of the Grounds and Buildings and the Memorials, Portraits and Busts of Rutgers College. Rutgers College Publication No. 11.
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When Joshua Edward died childless in 1837, he inherited Markree. Once established as manager, he began creating facilities for astronomy, which became Markree Observatory. In 1831 he bought for £1200 the largest object-glass which existed at the time. Made by the French optician Cauchoix, it had an aperture of 13⅓ inches () across and a focal length of 25 feet ().
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The difference in the two micrometer readings so obtained is the apparent separation or angular distance between the two stars. Heliometer by Carl Bamberg, Berlin, around 1880/90. Diameter of the lens 4.2 cm. Double image of the solar disk 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.
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It was earlier installed as an alt-az mount in the South dome at Greenwich, which in 1838 is where the then-new 6.7 inch aperture Sheepshanks refractor was installed. The telescope tube is 5 feet four inches long (about 1.6 meters). The focal length was the same for this telescope, with object glass being doublet of 4.1 inch (~10.4 cm) aperture. Shuckburgh placed an order with Ramsden for the telescope in 1781, and it was delivered for his observatory ten years later.
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This was described by Airy in his original work:Airy, G. B., "On the Diffraction of an Object-glass with Circular Aperture", Transactions of the Cambridge Philosophical Society, Vol. 5, 1835, p. 288. > The rapid decrease of light in the successive rings will sufficiently > explain the visibility of two or three rings with a very bright star and the > non-visibility of rings with a faint star. The difference of the diameters > of the central spots (or spurious disks) of different stars ... is also > fully explained.
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Edmund Halley communicated to the Royal Society his phase- determinations of the total solar eclipse of 3 May 1715. On 14 July 1715 Pound observed an occultation of a star by Jupiter, on 30 October an eclipse of the moon, and made, in 1716 and 1717, various planetary observations all with a fifteen-foot telescope. Huygens's 123-foot focal length object-glass, lent to Pound in 1717 by the Royal Society, was mounted by him in the grounds of Wanstead House on the maypole just removed from the Strand, and procured for the purpose by Sir Isaac Newton. The inconveniences of this instrument were commented upon by Joseph Crosthwait.
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One of Wheatstone's most ingenious devices was the 'Polar clock,' exhibited at the meeting of the British Association in 1848. It is based on the fact discovered by Sir David Brewster, that the light of the sky is polarised in a plane at an angle of ninety degrees from the position of the sun. It follows that by discovering that plane of polarisation, and measuring its azimuth with respect to the north, the position of the sun, although beneath the horizon, could be determined, and the apparent solar time obtained. 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.
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Rutgers named the building after New York City businessman, Daniel S. Schanck, who donated a large portion of the funds to construct and equip the observatory. The cost of cost of construction and equipment amounted to US$6,166 (2013: US$86,845.07), of which US$2,400 (2013: US$33,802.82) was donated by Schanck (1812–1872). Rutgers equipped the observatory with "a 6.5-inch equatorial refracting telescope, a meridian circle with four-inch object glass for transit observations, a sidereal clock, a mean solar clock...chronograph, repeating circle, and other instruments." The Schanck Observatory served as the university's first astronomical facility and was used to provide instruction to its students through the nineteenth and early twentieth century.
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The diffraction pattern resulting from a uniformly illuminated, circular aperture has a bright central region, known as the Airy disk, which together with the series of concentric rings around is called the Airy pattern. Both are named after George Biddell Airy. The disk and rings phenomenon had been known prior to Airy; John Herschel described the appearance of a bright star seen through a telescope under high magnification for an 1828 article on light for the Encyclopedia Metropolitana: Airy wrote the first full theoretical treatment explaining the phenomenon (his 1835 "On the Diffraction of an Object-glass with Circular Aperture"). Mathematically, the diffraction pattern is characterized by the wavelength of light illuminating the circular aperture, and the aperture's size.
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However, if the eye relief is too large it can be uncomfortable to hold the eye in the correct position for an extended period of time, for which reason some eyepieces with long eye relief have cups behind the eye lens to aid the observer in maintaining the correct observing position. The eye pupil should coincide with the exit pupil, the image of the entrance pupil, which in the case of an astronomical telescope corresponds to the object glass. Eye relief typically ranges from about 2 mm to 20 mm, depending on the construction of the eyepiece. Long focal-length eyepieces usually have ample eye relief, but short focal-length eyepieces are more problematic.
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In 1862 Foucault was made a member of the Bureau des Longitudes and an officer of the Legion of Honour. He became a member of the Royal Society of London in 1864, and member of the mechanical section of the Institute a year later. In 1865 he published his papers on a modification of James Watt's centrifugal governor; he had for some time been experimenting with a view to making its period of revolution constant and developing a new apparatus for regulating the electric light. Foucault showed how, by the deposition of a transparently thin film of silver on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye.
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Metal, paper, textiles, and other porouse materials require the use of cotton gloves to prevent oils from human skin tarnishing, staining, or otherwise damaging the object. Glass, glazed ceramics, and other potentially slippery surfaces should be handled using latex or vinyl gloves to prevent slippage. Prior to handling an object, it is also important to (1) take note of any structural weaknesses so that direct pressure on these areas may be avoided, (2) any clothing or accoutrements with the potential to scratch or catch on the object should be secured, and (3) the object's weight should be considered so that additional help may be requested when handling heavier objects. It is important to examine the types of materials to be used while handling the collection.
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In 1707 the astronomer James Pound became rector of Wanstead. In 1717 the Royal Society lent Pound Huygens's 123-foot focal length object- glass, which he set up in Wanstead Park. Pound's observations with it of the five known satellites of Saturn enabled Halley to correct their movements; and Newton employed, in the third edition of the Principia, his micrometrical measures of Jupiter's disc, of Saturn's disc and ring, and of the elongations of their satellites; and obtained from him data for correcting the places of the comet of 1680. Laplace also used Pound's observations of Jupiter's satellites for the determination of the planet's mass; and Pound himself compiled in 1719 a set of tables for the first satellite, into which he introduced an equation for the transmission of light.
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He resigned in 1863 as a direct result of the dismissal of Tappan by the University's regents and returned to Germany. Then, on the death of Sir William Rowan Hamilton in 1865, he accepted the post of Andrews Professor of Astronomy in the University of Dublin and Royal Astronomer of Ireland. 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. The first, second and third parts of the Astronomical Observations and Researches made at Dunsink contain the results of these labors, and include discussions of the distances of the stars α Lyrae, ο Draconis, Groombridge 1830, 85 Pegasi, and Bradley 3077, and of the planetary nebula H. iv. 37.
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Although enrolled at Cambridge University, he does not appear to have earned a degree. He was admitted as a sizar at Clare College in 1793, but may not have gained residence. An original proprietor of the Royal Institution, Pearson finished a planetarium in 1803 that illustrated Dr. Thomas Young's lectures. On 10 January 1810 Pearson was presented to the rectory of Perivale in Middlesex. On 15 March 1817, Lord-chancellor Eldon presented Pearson to the rectory of South Kilworth in Leicestershire. He acquired the Temple Grove School, a large private institution at East Sheen in 1810. After establishing an observatory there, he measured the diameters of the sun and moon during the partial solar eclipse of 7 September 1820, with one of John Dollond's divided object-glass micrometers. The foundation of the Astronomical Society of London (now known as the Royal Astronomical Society) was largely due to his efforts. In 1812 and 1816, he began development of the society that formally took shape during a meeting at the Freemasons' Tavern on 12 January 1820.
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