Note oo — BOOK IV.

The glorious Landskips, and other Objects that present themselves to the Eye, are manifestly painted on the Retina, and that not erect, but inverted as the Laws of Opticks require; and is manifest to the Eye from Monsieur Cartes’s Experiment, of laying bare the vitreous Humour on the back part of the Eye, and clapping over it a Bit of white Paper, or the Skin of an Egg; and then placing the fore-part of the Eye to the Hole of the Window of a darkned Room. By which means we have a pretty Landskip of the Objects abroad invertedly painted on the Paper, on the back of the Eye. But now the Question is, How in this Case the Eye comes to see the Objects erect? Monsieur Cartes’s Answer is, Notitia illius ex nullâ imagine pendet, nec ex ullâ actione ab objectis veniente, sed ex solo situ exiguarum partium cerebri, è quibus Nervi expullulant.——E.g. cogitandum in Oculo——situm capillamenti nervi optici——respondere ad alium quendam partis cerebri——qui facit ut Anima singula loca cognoscat, quæ jacent in rectâ, aut quasi rectâ lineæ; ut ita mirari non debeamus corpora in naturali situ videri, quamvis imago in oculo delineata contrarium habeat. Dioptr. c. 6. But our most ingenious Mr. Molyneux answereth thus, The Eye is only the Organ or Instrument, ’tis the Soul that sees by means of the Eye. To enquire then how the Soul perceives the Object erect, by an inverted Image, is to enquire into the Soul’s Faculties——But erect and inverted are only Terms of Relation to up and down; or farther from, or nigher to the Center of the Earth, in Parts of the same Thing.——But the Eye, or visive Faculty takes no notice of the internal Posture of its own Parts, but useth them as an Instrument only, contrived by Nature for the Exercise of such a Faculty.——Let us imagine, that the Eye (on its lower Part) receives an Impulse [by a Ray from the upper part of the Object] must not the visive Faculty be necessarily directed hereby to consider this Stroke, as coming from the top rather than the bottom [of the Object] and consequently be directed to conclude it the Representation of the top? Hereof we may be satisfied, by supposing a Man standing on his Head. For here, though the upper Parts of Objects are painted on the upper Parts of the Eye, yet the Objects are judged to be erect. What is said of Erect and Reverse, may be understood of Sinister and Dexter. Molyneux’s Dioptr. Nov. Part I. Prop. 28.