Dioptrics

...displayed, and he granted something to the one using the emissions of rays The "extramission" theory, which held that the eye sends out feeler-like rays to "touch" objects. out of respect for the opinion of antiquity—a pursuit irrelevant to philosophy. Therefore, I take these words from Pena’s own mouth, and I write back to him his own sentiment: I want a physicist who is not at all credulous, and for that reason, one experienced in optical demonstrations, who examines Euclid (he had said Vitello Witelo (c. 1230 – after 1280), a medieval natural philosopher whose work Perspectiva was the standard optical text before Kepler.) and other opticians accurately, and believes them only as much as he sees demonstrated by them. Euclid was a man second to none in learning and erudition, as his monuments show; but—as is the common lot of rising disciplines—he held preconceived opinions which he offered as axioms for demonstrations. One such opinion is that sight occurs through rays hastening from the eyes to the object seen, which is no more necessary than if you were to say vision occurs through the reception of rays. These things, I say, I think should be thrown back at Pena in this place. For it makes no difference to certain demonstrations which of these is true; and you see both used by me indiscriminately in Propositions 3 and 19. Yet this distinction must be noted: if we are dealing with the nature of a shining thing, it is useful for us to speak clearly, and to instill nothing other than the emissions of rays from shining points. But if we speak of the vision of shining things, and of the deceptions of sight, the deceptions themselves often invite us to speak almost captiously Kepler suggests that when describing an optical illusion, it is sometimes easier to use the language of "eye-rays" even if it is physically incorrect., and to use the language of emissions of rays from the eye, when in reality they are receptions of rays into the eye.

Why one thing is seen with two eyes: Pena correctly refutes the false reasoning of Vitello, but he incorrectly praises the equally false reasoning of Galen Claudius Galenus (129–c. 216 AD), the influential Greek physician.. Galen used optical terms in a way that hardly follows optical laws—as if the pyramids of vision formed by the very act of seeing, and continued from the seen thing as if from a common base to the eyes, became something real and corporeal, which could be twisted away from the seen object by the distortion of the eye. You will find the true cause below in Proposition 62.

Aristotle saw long ago that the explanation of the Halo, the Rainbow, Parhelia Parhelia: also known as "sun dogs," these are bright spots that appear on either side of the sun due to ice crystals in the atmosphere., and Paraselenae Paraselenae: "moon dogs," the lunar equivalent of sun dogs. must be sought from the discipline of Optics; nor those things which...