Laurent Cassegrain
Encyclopedia
Laurent Cassegrain was a Catholic priest who is notable as the probable inventor of the Cassegrain reflector
, a folded two mirror reflecting telescope
design.
) on September 1, 1693.
The cassegrain reflector is a reflecting telescope design that solved the problem of viewing an image without obstructing the primary mirror
by using a convex secondary mirror
on the optical axis to bounce the light back through a hole in the primary mirror thus permitting the light to reach an eyepiece.
It first appeared in the eighth edition of the 17th century French science journal Recueil des mémoires et conférences concernant les arts et les sciences, published by Jean-Baptiste Denys
in April 25, 1672. In that edition is found an extract from a letter written by M. de Bercé, writing from Chartres
, where he acted as a representative for the Académie des sciences —scholars of Chartres. M. de Bercé reported on a man named Cassegrain who had written a letter on the megaphone
with an attached note describing a new type of reflecting telescope, the Cassegrain reflector, where a secondary convex mirror is suspended above a primary concave mirror. This was around the time of the publication of the construction of the first practical reflecting telescope, Isaac Newton
's newtonian reflector. On June 13, 1672, Christiaan Huygens wrote about the Cassegrain design and critiqued it harshly, maybe because Huygens felt Newton's design was being "imperiled" by this alternative. Whatever the motives, the storm of controversy that followed had one lasting effect: Cassegrain's name was forgotten.
The identity of this "Cassegrain" has had many theories. His only known publication was the letter on the megaphone/reflecting telescope in the April 25, 1672 Recueil des mémoires et conférences concernant les arts et les sciences. For a long time, reference works were forced to report his first name as "not conclusively known". The Encyclopaedia Britannica (15th edition, 1974), for example, only goes as far as listing "Cassegrain, N." (this, in turn, seems to come from Ferdinand Hoefer
's Nouvelle biographie générale, Paris
, 1855). Other sources have suggested the "N." stood for Nicolas. Some sources (such as La grande encyclopédie, 9, 696) claim his name to be Guillaume, a metal-caster and sculptor who is mentioned in the accounts of king Louis XIV
's buildings between 1684 and 1686, and also in a Paris notarized act from 1693. Another name put forward is Jacques, a chirurgeon or doctor, who is mentioned in the Mémoires de l’Académie des sciences as having found, in 1691, a piece of magnet in the steeple of Chartres Cathedral, then being repaired after being damaged by inclement weather.
In 1997 two French astronomers, Andre Baranne and Francois Launay, after a long and meticulous investigation including a search for unpublished manuscripts and the analysis of parish registers in the places where Cassegrain lived (Chartres first and then Chaudon
, near Nogent-le-Roi
), identified Laurent Cassegrain as the most likely candidate.
The crater Cassegrain
on the Moon
is named after him, even though his true identity was not known at the time of the naming.
Cassegrain reflector
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas....
, a folded two mirror reflecting telescope
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
design.
Biography
Laurent Cassegrain was born in the region of Chartres around 1629 and was the son of a surgeon. It is unknown what his education was but he was a priest and professor by 1654. He may have been interested in acoustics, optics and mechanics. At the time of his death he was working as a teacher giving science classes at the Collège de Chartres, a French lycée, i.e., a high-school like institution. He died at Chaudon (Eure-et-LoirEure-et-Loir
Eure-et-Loir is a French department, named after the Eure and Loir rivers.-History:Eure-et-Loir is one of the original 83 departments created during the French Revolution on March 4, 1790 pursuant to the Act of December 22, 1789...
) on September 1, 1693.
Connection with the cassegrain reflector
Primary mirror
A primary mirror is the principal light-gathering surface of a reflecting telescope.-Description:The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal , or in later telescopes, glass or other material coated with a reflective layer...
by using a convex secondary mirror
Secondary mirror
A secondary mirror is the second deflecting or focusing mirror element in a reflecting telescope. Light gathered by the primary mirror is directed towards a focal point typically past the location of the secondary. Secondary mirrors in the form of an optically flat diagonal mirror are used to...
on the optical axis to bounce the light back through a hole in the primary mirror thus permitting the light to reach an eyepiece.
It first appeared in the eighth edition of the 17th century French science journal Recueil des mémoires et conférences concernant les arts et les sciences, published by Jean-Baptiste Denys
Jean-Baptiste Denys
Jean-Baptiste Denys was a French physician notable for having performed the first fully documented human blood transfusion. He studied in Montpellier and was the personal physician to King Louis XIV.- Attempts to transfuse blood :...
in April 25, 1672. In that edition is found an extract from a letter written by M. de Bercé, writing from Chartres
Chartres
Chartres is a commune and capital of the Eure-et-Loir department in northern France. It is located southwest of Paris.-Geography:Chartres is built on the left bank of the Eure River, on a hill crowned by its famous cathedral, the spires of which are a landmark in the surrounding country...
, where he acted as a representative for the Académie des sciences —scholars of Chartres. M. de Bercé reported on a man named Cassegrain who had written a letter on the megaphone
Megaphone
A megaphone, speaking-trumpet, bullhorn, blowhorn, or loud hailer is a portable, usually hand-held, cone-shaped horn used to amplify a person’s voice or other sounds towards a targeted direction. This is accomplished by channelling the sound through the megaphone, which also serves to match the...
with an attached note describing a new type of reflecting telescope, the Cassegrain reflector, where a secondary convex mirror is suspended above a primary concave mirror. This was around the time of the publication of the construction of the first practical reflecting telescope, Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
's newtonian reflector. On June 13, 1672, Christiaan Huygens wrote about the Cassegrain design and critiqued it harshly, maybe because Huygens felt Newton's design was being "imperiled" by this alternative. Whatever the motives, the storm of controversy that followed had one lasting effect: Cassegrain's name was forgotten.
The identity of this "Cassegrain" has had many theories. His only known publication was the letter on the megaphone/reflecting telescope in the April 25, 1672 Recueil des mémoires et conférences concernant les arts et les sciences. For a long time, reference works were forced to report his first name as "not conclusively known". The Encyclopaedia Britannica (15th edition, 1974), for example, only goes as far as listing "Cassegrain, N." (this, in turn, seems to come from Ferdinand Hoefer
Ferdinand Hoefer
Jean Chrétien Ferdinand Hoefer was a German-French physician and lexicographer. He is now known for his many works on the history of science.-Selected works:...
's Nouvelle biographie générale, Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
, 1855). Other sources have suggested the "N." stood for Nicolas. Some sources (such as La grande encyclopédie, 9, 696) claim his name to be Guillaume, a metal-caster and sculptor who is mentioned in the accounts of king Louis XIV
Louis XIV of France
Louis XIV , known as Louis the Great or the Sun King , was a Bourbon monarch who ruled as King of France and Navarre. His reign, from 1643 to his death in 1715, began at the age of four and lasted seventy-two years, three months, and eighteen days...
's buildings between 1684 and 1686, and also in a Paris notarized act from 1693. Another name put forward is Jacques, a chirurgeon or doctor, who is mentioned in the Mémoires de l’Académie des sciences as having found, in 1691, a piece of magnet in the steeple of Chartres Cathedral, then being repaired after being damaged by inclement weather.
In 1997 two French astronomers, Andre Baranne and Francois Launay, after a long and meticulous investigation including a search for unpublished manuscripts and the analysis of parish registers in the places where Cassegrain lived (Chartres first and then Chaudon
Chaudon
Chaudon is a commune in the Eure-et-Loir department in northern France.-Population:-References:*...
, near Nogent-le-Roi
Nogent-le-Roi
Nogent-le-Roi is a commune in the department of Eure-et-Loir in the Centre region in northern France.It is located some 20 kilometres north of Chartres and a shorter distance to the southeast of Dreux.-Population:-Royal deaths:...
), identified Laurent Cassegrain as the most likely candidate.
The crater Cassegrain
Cassegrain (crater)
Cassegrain is a lunar impact crater that is located on the far side of the Moon, beyond the southeastern limb. It lies to the southeast of the larger crater Lebedev, and to the northeast of the comparably-sized Priestley....
on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
is named after him, even though his true identity was not known at the time of the naming.
External links
- Chaudon web page on Laurent Cassegrain (in French)