Dioptrics

...of rays closest to the body, from where such a tail begins.

Although indeed this reasoning of Pena Jean Pena (1528–1558), a French mathematician whose preface to Euclid’s Optics challenged Aristotelian physics. concerning the effect of the comet is doubtful, as I said; yet I am so far from judging it as something to be utterly despised, that I would rather recommend it to everyone as most noble, and a theory from which I believe the most hidden secret of Nature concerning the tails of comets can be drawn out. I wrote something on this matter in the German description of the Comet that shone in the year 1607 Now known as Halley's Comet; Kepler published a report on it in 1608.: which I had also prepared in Latin with a most beautiful demonstration of the comet’s rectilinear rectilinear: moving in a straight line. path through the depths of the ether; but the printer cheated my expectation, and the little book remains in my desks, awaiting another opportunity.

Pena mentions the sixth utility of Optics in tearing down the opinion of the Aristotelians concerning the Galaxy The Milky Way.: and he teaches from Optics that the Galaxy is poured around within the ether itself, very far above the moon: since it presents no change of position Original: permutationem loci. This refers to parallax; if an object is close to Earth, it appears to shift against the background when viewed from different locations. Because the Milky Way does not shift, it must be very distant. relative to the fixed stars through different locations on earth and different positions relative to the horizon. Without a doubt, those who have hitherto looked up to Aristotle’s Meteorology Aristotle argued the Milky Way was a phenomenon in the upper atmosphere, like a slow-burning fire. and held it in high regard will recognize this great benefit of optics. Although those things which Galileo discovered concerning the Galaxy by the help of the telescope Original: perspicilli. In 1610, Galileo used his telescope to prove the Milky Way was composed of countless individual stars. will henceforth render this reasoning of Pena’s superfluous.

There follow in Pena’s preface some unsightly points, by which I ask that the student of Optics not be moved. Vitellio Witelo (c. 1230–1280/1314), a Polish friar and physicist whose work Perspectiva was a standard textbook on optics. most truly established that vision occurs by the reception of rays; I have confirmed this by the most evident experiments. Great was the boasting of the Opticians against the Aristotelians who defend the emission of rays The "extramission" theory, which wrongly proposed that the eyes send out light to "feel" objects. because of their own agreement among themselves. It is therefore to be lamented that this glory of the Opticians is stained by Pena’s contrary testimony, especially since Pena himself is an Optician and recommends Optics in that very preface. Yet let the cultivator of philosophy consider that the investigation of hidden things does not belong to a single century; often truth, coming as if by stealth into sight, offended by the negligence of philosophers, suddenly withdraws itself again, not deigning to grant a mere sight of itself to men, unless they are dutiful and industrious. Add to this that Pena to Euclid, whom he had translated anew in that little book...