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(fl. Chartres, France, 1672),


Virtually all that is known about Cassegrain is that he conceived the arrangement of telescope mirrors that bears his name. So obscure is his life that even his identity itself is in question. One tradition credits him with a professorship of physics at the Collège deChartres; another identifies him with a man who served Louis XIV as a sculptor and founder.

Cassegrain’s appearance on the scientific scene was occasioned by a memoir on the megaphone published in February 1672. Having his own thoughts on the subject, Cassegrain quickly submitted them to the Paris Academy of Sciences. In the covering letter, his intermediary, one Henri de Bercé of Chartres, mentioned that Cassegrain had also conceived a refracting telescope, different from the one recently described by Newton in the Philosophical Transactions. The design consisted of a convex secondary mirror placed so as to intercept the rays from the objective before they arrive at the focus and to reflect them straight back to the eyepiece through a hole in the objective. It was set forth briefly in the proceedings for 25 April 1672. The accompanying implications of priority and claims of superiority were easily dispatched by Newton, who pointed out that the idea was only a minor modification of one published by James Gregory in 1663. The real virtue of the design—partial cancellation of the spherical aberrations introduced by the two mirrors—was established by Ramsden a century later. Since that time, the Cassegrain focus has been very popular in the construction of large refractors.


I. Original Works. His writings are “Extrait d’une lettre de M. de Bercéécrite de Chartres à 1’auteur de ces mémoires, touchant la trompette à parler de loin, dont on a donnél’explication dans le second mémoire; & to uchant la nouvelle lunette de M. Newton, dont it a estéparlé dansle troisième,” in Journal des savants, 3 (1672–1674),121–123; and “Extrait d’une lettre écrite de Chartres par Monsieur Cassegrain; Sur les proportions des trompettes à parler de loin.” ibid, 131–137.

II. Secondary Literature See Isaac Newton, “Some Considerations Upon Part of a Letter of M. de Bercé, Concerning His Reflecting Telescope Pretended To Be Improv’d by M. Cassegrain,” in Philosophical Transactions of the Royal Society of London. 7 (1672), 4056–4059; and “N. Cassegrain,” in Hoefer, Nouvelle biographic générale (Paris, 1855). See also “Guillaume Cassegrain,” in La grande encyclopédic, IX, 696; since this Cassegrain appears in the state records from 1666 to 1684, he at least constitutes a candidate who is known to have actually existed. His identification as the writer of the material in question, however, appears to be either entirely circumstantial or heavily dependent on several references to fondeurs decloches in the discussion of the megaphone.

Victor E. Thoren

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