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Fresnel lens

A Fresnel lens is a type of composite compact lens developed by French physicist Augustin-Jean Fresnel for lighthouses. It has been called "the invention that saved a million ships."The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances; the idea of creating a thinner, lighter lens in the form of a series of annular steps is attributed to Georges-Louis Leclerc, Comte de Buffon. Whereas Buffon proposed grinding such a lens from a single piece of glass, the Marquis de Condorcet proposed making it with separate sections mounted in a frame. French physicist and engineer Augustin-Jean Fresnel is most given credit for the development of the multi-part lens for use in lighthouses.

According to Smithsonian magazine, the first Fresnel lens was used in 1823 in the Cordouan lighthouse at the mouth of the Gironde estuary. Scottish physicist Sir David Brewster is credited with convincing the United Kingdom to adopt these lenses in their lighthouses; the Fresnel lens reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections. An ideal Fresnel lens would have an infinite number of sections. In each section, the overall thickness is decreased compared to an equivalent simple lens; this divides the continuous surface of a standard lens into a set of surfaces of the same curvature, with stepwise discontinuities between them. In some lenses, the curved surfaces are replaced with flat surfaces, with a different angle in each section; such a lens can be regarded as an array of prisms arranged in a circular fashion, with steeper prisms on the edges, a flat or convex center. In the first Fresnel lenses, each section was a separate prism.'Single-piece' Fresnel lenses were produced, being used for automobile headlamps, brake and turn signal lenses, so on.

In modern times, computer-controlled milling equipment might be used to manufacture more complex lenses. Fresnel lens design allows a substantial reduction in thickness, at the expense of reducing the imaging quality of the lens, why precise imaging applications such as photography still use larger conventional lenses. Fresnel lenses are made of glass or plastic. In many cases they are thin and flat flexible, with thicknesses in the 1 to 5 mm range. Modern Fresnel lenses consist of all refractive elements; however many of the lighthouses have both refracting and reflecting elements, as shown in the photographs and diagram. That is, the outer elements are sections of reflectors while the inner elements are sections of refractive lenses. Total internal reflection was used to avoid the light loss in reflection from a silvered mirror. Fresnel produced six sizes of lighthouse lenses, divided into four orders based on their size and focal length. In modern use, these are classified as first through sixth order.

An intermediate size between third and fourth order was added as well as sizes above first order and below sixth. A first-order lens has a focal length of a maximum diameter 2590 mm high; the complete assembly is 1.8 m wide. The smallest has a focal length of an optical diameter 433 mm high; the largest Fresnel lenses are called hyperradiant Fresnel lenses. One such lens was on hand when it was decided to outfit the Makapuu Point Light in Hawaii. Rather than order a new lens, the huge optic construction, 3.7 metres tall and with over a thousand prisms, was used there. There are two main types of Fresnel lens: non-imaging. Imaging Fresnel lenses use segments with curved cross-sections and produce sharp images, while non-imaging lenses have segments with flat cross-sections, do not produce sharp images; as the number of segments increases, the two types of lens become more similar to each other. In the abstract case of an infinite number of segments, the difference between curved and flat segments disappears.

Spherical A spherical Fresnel lens is equivalent to a simple spherical lens, using ring-shaped segments that are each a portion of a sphere, that all focus light on a single point. This type of lens produces a sharp image, although not quite as clear as the equivalent simple spherical lens due to diffraction at the edges of the ridges. Cylindrical A cylindrical Fresnel lens is equivalent to a simple cylindrical lens, using straight segments with circular cross-section, focusing light on a single line; this type produces a sharp image, although not quite as clear as the equivalent simple cylindrical lens due to diffraction at the edges of the ridges. Spot A non-imaging spot Fresnel lens uses ring-shaped segments with cross sections that are straight lines rather than circular arcs; such a lens does not produce a sharp image. These lenses have application such as focusing sunlight on a solar panel. Fresnel lenses may be used as components of Köhler illumination optics resulting

St Mary's Church, Cardigan

St Mary's Church is an Anglican church located in Cardigan, Wales. In 1428, with the disappearance of Holy Trinity Church, St Mary's formally became the parish church of Cardigan, it is located close to the Cardigan Priory. It is a Grade II listed building. St. Mary's was both the priory church of the medieval Benedictine Cardigan Priory and a parish church, it continues as a parish church. While the church was a twelfth century Benedictine foundation, the present building dates from the fourteenth century, although rebuilt. "It's blocked south door is thought is thought to have been associated with the former priory buildings." In the thirteenth century St Mary's Priory church was the site of the shrine of Our Lady of the Taper, demolished at the Dissolution of the Monasteries. The porch was rebuilt in 1639. In 1705, the tower collapsed; the stained glass window in the east wall of the chancel was installed n 1924, depicts the Crucifixion with the Virgin Mary and St John. Surviving fragments of fifteenth century glass are set in the upper tracery lights.

Dr David Rowlands, Inspector of H. M. Hospitals and Fleets, Royal Navy, died in 1846, he is commemorated by a memorial tablet inside the church. Parish website

Jesse Burton Harrison

Jesse Burton Harrison was an American anti-slavery lawyer and author. Jesse Burton Harrison was born in 1805 in Virginia, his father, Samuel Jordan Harrison, was a well-to-do tobacco merchant, friend of Thomas Jefferson, who had helped to build the University of Virginia. "As a young man he was a habitué at Monticello. James Madison was his patron." Jesse was educated at Hampden–Sydney College and at the Harvard Law School, where he was influenced by George Ticknor. He practiced law in Lynchburg. Failing to obtain an appointment as professor at the University of Virginia, in 1828 he traveled to Europe with a letter of introduction from Secretary of State Martin Van Buren, he met Lafayette, Benjamin Constant and Goethe, spent a year studying at the University of Göttingen. He moved to New Orleans, where he helped to found the Louisiana Historical Society, edited the Louisiana Law Reports and the Whig newspaper Louisiana Advertiser. Harrison delivered a series of literary addresses and in the late 1820s, began publicly supporting anti-slavery thought.

He published an appeal on behalf of the American Colonization Society in 1827. Most he wrote a response to Thomas Roderick Dew's pro-slavery essay, Review of the Debates in the Virginia Legislature, 1831-2. Harrison died of yellow fever in 1841 in New Orleans, he was the father of Burton Harrison, a Confederate official and lawyer, the grandfather of Fairfax Harrison and Francis Burton Harrison. His wife was the former Frances Anne Brand. Harrison, Jesse Burton. "English Civilization". In O'Brien, Michael. All Clever Men, Who Make Their Way: Critical Discourse in the Old South. University of Georgia Press. Pp. 55–88. ISBN 9780820314907