Lot n° 232
Estimation :
15000 - 20000
EUR
Result with fees
Result
: 21 760EUR
Pierre GASSENDI (1592-1655) mathematician,... - Lot 232 - Ader
Pierre GASSENDI (1592-1655) mathematician, astronomer and philosopher. L.A.S. " P. Gassendus ", Aix-en-Provence nones (5) of December 1636, to Gabriel Naudé in Rome; 3 pages small in-fol. filled with a small tight handwriting with two pen drawings, address " A Monsieur Naudé Coner & medecin ordre du Roy à Rome ", in Latin.
Extremely rare, long and very important scientific letter on the observation of the sun, the corpuscular theory of light and retinal vision, illustrated with two drawings.
This letter, signed at the top " P. Gassendus G. Naudæo suo S.", with some erasures and corrections, was published by Gassendi himself : it is indeed the first (p. 1-8) of his treatise De apparente magnitudine Solis humilis et sublimis epistolae quatuor, in quibus complura physica opticaque problemata proponuntur et explicantur (Paris, Hacqueville, 1642), which is composed of 4 letters : this one to Naudé, to Liceti (August 13, 1640), to Boulliau (December 28, 1640) and to Chapelain (January 13, 1641). There are very slight variations with the printed text, which divides the letter into VII numbered sequences, these numbers not appearing on the autograph.
[The writer Gabriel Naudé (1600-1653), the future librarian of Mazarin, had obtained his doctorate of medicine in Padua in 1633, and received the title of ordinary physician of Louis XIII; he was then in Rome as librarian of cardinal Bagni, and was elected member of the Accademia degli Umoristi. Earlier, in Paris, in the entourage of President de Mesmes, whose librarian he was, and of Jacques Dupuy, he had made friends with Gassendi, like him an eminent figure of the scholarly libertine of his time].
Naudé had reproached Gassendi for being obscure by saying that the sun, when it is close to the horizon, seems greater than in the heights of the sky. To imagine this is difficult and must be proved by an autopsia [observation by oneself]: "Dixeram Solem horizonti vicinum, ac inter vapores degentem conspici majorem quam in æthere sublimi ac puro". He had observed that the sun at the horizon cast a larger shadow than when it was rising, and had been able to verify this by the example of the moon, thanks to unquestionable experiments. But there is still a difficulty: if the shadow is bigger, while the sun descends to the horizon, the sun must therefore appear smaller; which is opposed to the previous observation. But when he says cast a larger shadow, he does not mean longer: it is certain that a longer shadow is produced above the horizon line by the low sun, and shorter when it is high. Rather, he means larger, observed from the latitude and in the transverse diameter: "Itaque cum dico in primis majorem umbram proiici, longiorem non intelligo: certum est enim umbram longiorem ab humili Sole creari supra horizontis planitiem, ac breviorem ex edito. Intelligo potius crassiorem, sive secundum latitudinem, & in diametro transversa spectatam ". He tries to explain the apparent differences in the size of the sun and the moon at different times by referring to the visual experience produced by light phenomena. Thus these bodies appear larger at the horizon than at their peak, because the pupil dilates due to the differential exposure to light at the horizon.
He explains by a double drawing: let A be an opaque body, or B the same, the larger light source at D, the smaller at E, both at the same distance from the same plane F G. He thus demonstrates that the shadow of A, created by D, received at F, is narrower than the shadow of B, created by E, received at G. So if we suppose that the Sun at the horizon is D, and when it is elevated is E, when it is larger, it must cast a smaller shadow as seen at F, and if it is smaller a larger shadow as seen at G. It is the same thing for the Moon...
He then draws the instrument he used to calculate the diameters of the Sun and the Moon: "Ea refert valdè quidem simplex, sed adpositum tamen Organum, quo Solis, ac Lunæ aucupari diametros soleo". And he explains: "Nempe H, I, repræsentatam Trabeculam quatuor propemodum orgyiarum, sive toisarum parisiensium. KLMN & OPQR sunt duo æqualia pinnacidia dimidium prope pedis lata, ac ipsi plano Trabeculæ, ejusque extremis ad normam erecta "... HI is a smooth board of 4 orgyiarum long, or Parisian toises; KLMN & OPQR are two equal pinnacles separated by about half a foot, placed at each end. He builds this board in this way, and props it up so that the board is placed behind the sun's ray, the KLMN pinnule in the upper part, the sun directs its shadow on the lower part OPQR. The sun being larger than the upper pinnule, it is necessary that its shadow decreases when it rises, and when it reaches the lower pinnule, it is necessary that its shadow decreases.
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