Laurent Cassegrain (; – 1 September 1693) was a

Catholic priest The priesthood is the office of the ministers of religion, who have been commissioned ("ordained") with the Holy orders of the Catholic Church. Technically, bishops are a priestly order as well; however, in layman's terms ''priest'' refers only ...

who is notable as the probable inventor of the Cassegrain reflector, a folded two-mirror

reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...

design.

Biography

Laurent Cassegrain was born in the region of Chartres around 1629 and was the son of Mathurin Cassegrain and Jehanne Marquet. 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-Loir Eure-et-Loir (, locally: ) is a French department, named after the Eure and Loir rivers. It is located in the region of Centre-Val de Loire. In 2019, Eure-et-Loir had a population of 431,575.primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective meta ...

by using a convex

secondary mirror A secondary mirror (or secondary) 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 mirro ...

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 (1643 – 3 October 1704) was a French physician notable for having performed the first fully documented human blood transfusion, a xenotransfusion. He studied in Montpellier and was the personal physician to King Louis ...

on 25 April 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 A megaphone, speaking-trumpet, bullhorn, blowhorn, or loudhailer is usually a portable or hand-held, cone-shaped acoustic horn used to amplify a person's voice or other sounds and direct it in a given direction. The sound is introduced int ...

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 Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a " natural philosopher"), widely recognised as one of the grea ...

Newtonian reflector The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newton' ...

. On 13 June 1672 Christiaan Huygens wrote about the Cassegrain design and critiqued it harshly, maybe because Huygens felt Newton's design was being "imperilled" 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 25 April 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 ''Encyclopædia Britannica'' (15th edition, 1974), for example, only goes as far as listing "Cassegrain, N." (this, in turn, seems to come from

Ferdinand Hoefer Jean Chrétien Ferdinand Hoefer (German: ''Ferdinand Höfer'', 21 April 1811, Döschnitz – 4 May 1878) was a German-French physician and lexicographer. He is now known for his many works on the history of science. Selected works *''Élément ...

's ''Nouvelle biographie générale'',

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), ma ...

, 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 , house = Bourbon , father = Louis XIII , mother = Anne of Austria , birth_date = , birth_place = Château de Saint-Germain-en-Laye, Saint-Germain-en-Laye, France , death_date = , death_place = Palace of Ver ...

's buildings between 1684 and 1686, and also in a Paris notarized act from 1693. Another name put forward is Jacques, a

chirurgeon In modern medicine, a surgeon is a medical professional who performs surgery. Although there are different traditions in different times and places, a modern surgeon usually is also a licensed physician or received the same medical training as ...

(i.e., surgeon) 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, André Baranne and Françoise 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 () is a commune in the Eure-et-Loir department in northern France. Population See also *Communes of the Eure-et-Loir department The following is a list of the 365 communes of the Eure-et-Loir department of France. The communes co ...

, near

Nogent-le-Roi Nogent-le-Roi () is a Communes of France, commune in the Departments of France, department of Eure-et-Loir in the Centre-Val de Loire Regions of France, region in France. It is located 27 kilometres north of Chartres and 18 kilometres south-east ...

), identified ''Laurent Cassegrain'' as the most likely candidate. The crater Cassegrain on the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

is named after him, even though his true identity was not known at the time of the naming.

Notes

External links

Chaudon web page on Laurent Cassegrain
(in French)

References

* Christiaan Huygens, ''Réflexions sur la description d'une lunette publiée sous le nom de M. Cassegrain'' (Letter #1892, addressed to Jean Gallois)
''Œuvres complètes'', vol. 7, pp. 189-191
1888.
André Baranne and Françoise Launay, ''Cassegrain: a famous unknown of instrumental astronomy''
Journal of Optics, 1997, vol. 28, no. 4, pp. 158–172(15) {{DEFAULTSORT:Cassegrain, Laurent 1629 births 1693 deaths 17th-century French Roman Catholic priests Telescope types French scientific instrument makers Catholic clergy scientists People from Chartres