Physiologia Epicuro-Gassendo-Charltoniana, or, A fabrick of science natural, upon the hypothesis of atoms founded by Epicurus repaired [by] Petrus Gassendus ; augmented [by] Walter Charleton ...

PARADOX.

That the Eye sees no more at one prospect then at another: or, that the Eye beholds as much when it looks on a shilling,* 1.1 or any other object of as small diameter, as when it speculates a Mountain, nay the whole Heaven.

Which though obscure and despicable at first planting, will yet require no more time to grow up to a firm and spreading truth, than while we investigate the Reasons of Two Cozen-German optical Phaeno∣mena's.

(1) Why an Object appears not only greater in dimensions, but more distinct in parts, when lookt upon near at hand; than afarr off?

(2) Why an Object, speculated through a Convex Glass, appears both larger and more distinct; than when beheld only with eye: but through a Concave, both Smaller, and more confused?

* 1.2To the solution of the First, we are to reflect on some of the praecedent Assumptions. For, since every Visible diffuseth rayes from all points of it superfice, into all regions of the medium, according to the second Assumption; and since the superfice of the most seemingly smooth and polite body, is variously interspersed with Asperities, from the various faces whereof, in∣numerable rayes are emitted, tending according their lines of Direction, in∣to all points of medium circularly; according to the first Assumption; and since those swarms of Emanations must be ••o much the more Dense and Congregate, by how much the less they are elongated from their fountain, or body exhalant; and è Contra, so much the more Rare and Disgregate, by how much farther they are deduced, according to the third Assumption: Therefore, by how much nearer the eye shall be to the object by so much a greater number of Rayes shall it receive from the various parts thereof, and the particles of those parts; and è Contra: and Consequently by how much a greater number of rayes are received into the pupill of the eye, by so much greater do the dimensions of the object, and so much the more distinct do the parts of it superfice appear. For it is axiomatical among the Masters of the Opticl••s, and most perfectly demonstrated by Scheinerus (in lib. 2. Fundament. Optic. part. 1. cap. 13.) that the Visive Axe consisteth not of one single raye, but of many concurring in the point of the pyramid, ter∣minated in the concave of the Retina Tunica: and as demonstrable, that those rayes only concurr in that conglomerated stream, which enters the Pupil, that are emitted from the parts of the object directly obverted unto it; all others ••ending into other quarters of the medium. And hence is it, that the image of a remote object, consisting of rayes (which though stream∣ing from distant parts of the superfice thereof, do yet, by reason of their concurse in the retused point of the visive Pyramid, represent those parts as Conjoyned) thin and less united, comparatively; those parts must appear as Contiguou•• in the visifical Representation, or Image, which are really In∣contiguous or seperate in the object: and upon consequence, the object

must be apprehended as Contracted, or Less, as consisting of fewer parts; and also Confused, as consisting of parts not well distinguisht. This may be truly, though somewhat grosly, Exemplified in our prospect of two or three Hills situate at large distance from our eye, and all included in the same Visive Hemisphere; for, their Elongation from the Eye makes them ap∣pear Contiguous, nay one and the same Hill, though perhaps they are, by more then single miles, distant each from other: or, when from a place of eminence we behold a spacious Campania beneath, and apprehend it to be an intire Plane; the Non-apparence of those innumerable interjacent Fos∣ses, Pits, Rivers, &c. deprest places, imposing upon the sense, and exhi∣biting it in a smooth continued plane.

And to the solution of the second Problem,* 1.3 a concise enquiry into the Causes of the different Effects of Concave and Convex Perspicils, in the representation of Images Visible, is only necessary. A Concave Lens, whether Plano-concave, or Concave on both sides, whether it be the segment of a great, or small Circle, projects the Image of an Object, on a paper set at convenient distance from the tube that holds it, Confused and insincere; because it refracts the rayes there∣of even to Disgregation, so that never uniting again, they are trans∣mitted in divided streams and cause a chaos, or perpetual confusi∣on. On the Contrary, a Convex Lens refracts the rayes before divided, even to a Concurse and Union, and so makes that Image Distinct and Ordinate, which at its incidence thereon was confused and inordinate. And so much the more perfect must every Convex Lens be, by how much greater the Sphere is, of which it is a Secti∣on. For, as Kircher well observes (in Magia parastatica.) if the Lens be not only a portion of a great sphere, V. Gr. such a one, whose diametre contains twenty or thirty Roman Palms; but hath its own diametre consisting of one, or two palmes: it will represent objects of very large dimensions, with so admirable similitude, as to inform the Visive Faculty of all its Colours, Parts, and other discoverables in it superfice. Of which sort are those excellent Glasses, made by that famous Artist, Eustachio Divini, at Rome; by the help whereof the Painters of Italy use to draw the most exquisite Choro∣graphical, Topographical, and Prosopographical Tables, in the World. This Difference betwixt Concave and Convex Perspicils is thus stated by Kircher (Art. Magnae Lucis & Umbrae▪ lib. 10. Magiae part. 2. Sect. 5.) Hinc patet differentia lentis Conve••ae & Concavae; quod illa confusam speciem acceptam transmissamque semper distinguit, & optimè ordinat: ••lla verò eandem perpetuo confundit; unde officium lentis Convexae est, easdem confusè accept is, in debita distantia, secundum suam potentiam, distinguere & ordinare. And by Scheine∣rus (in Fundam. Optic. lib. 3. part. 1. cap. 11.) thus; Licet in vitro quocunque refractio ad perpendicularem semper accidat, quia ta∣men ipsum superficie cava terminatur, radij in aerem egressi potius dis∣perguntur, quàm colliguntur: cujus contrarium evenit vitro Convexo, ob contrariam extremitatem. Rationes sumuntur à Refractionibus in di∣versa tendentibus, vitri Convexi & Concavi, ob contrarias Extremitatum configurationes. Concavitas enim radios semper magis divergit: sicut Con∣vexitas amplius colligit, &c.

Now, to draw these lines home to the Centre of our problem; since the Rayes of a Visible Image trajected through a Convex Perspicil, are so refracted, as to concurr in the Visive Axe: it is a clear consequence, that therefore an object appears both larger in dimensions, and more distinct in parts, when speculated through a Convex Glass, than when lookt upon on∣ly with the Eye; because more of the rayes are, by reason of the Con∣vexity of its extreme obverted to the object, conducted into the Pupil of the Eye, than otherwise would have been. For, whereas some rayes pro∣ceeding from those points of the object, which make the Centre of the Base of the Visive Pyramid, according to the line of Direction, incurr into the Pupil; others emanant from other parts circumvicine to those central ones, fall into the Iris; others from other parts circumvicine fall upon the eye∣lids; and others from others more remote, or nearer to the circumference of the Base of the Pyramid, strike upon the Eyebrows, Nose, Forehead, and other parts of the face: the Convexity of the Glass causeth, that all those rayes, which otherwise would have been terminated on the Iris, eye∣lids, brows, nose, forehead, &c. are Refracted, and by refraction deflected from the lines of Direction, so that concurring in the Visive Axe, they en∣ter the Pupil of the Eye in one united stream, and so render the Image im∣prest on the Retina Tunica, more lively and distinct, and encreased by so many parts, as are the rayes superadded to those, which proceed from the parts directly confronting the Pupil. On the Contrary; because an Image trajected through a Concave Perspicill, hath its rayes so refracted, that they become more rare and Disgregate: the object must therefore seem less in dimensions, and more confused in parts; because many of those rayes, which according to direct tendency would have insinuated into the Pupill, are diverted upon the Iris, Eyelids, and other circumvicine parts of the face.

Here opportunity enjoyns us to remember the duty of our Profession, nor would Charity dispense,* 1.4 should we, in this place, omit to prescribe some General Directions for the Melioration of sight, or natively, or acci∣dentally imperfect. The most common Diminutions of Sight, and those that may best expect relief from Dioptrical Aphorisms, and the use of Glasses; are only Two: Presbytia, and Myopia. The First, as the word im∣ports, being most familiar to old men, is (Visus in perspiciendis object is pro∣pinquis obscuritas; in remotis verò integrum acumen) an imperfection of the sight, by reason whereof objects near hand appear obscure and confused, but at more distance, sufficiently clear and distinct. The Cause hereof generally, is the defect of due Convexity on the outside of the Chrystal∣line Humor; arising either from an Error of the Conformative Faculty in the Contexture of the parts of the Eye, or (and that mostly) from a Con∣sumption of part of the Chrystalline Humour by that Marasmus, Old Age: which makes the common Base of the Image Visible to be traje∣cted so far inwards, as not to be determined precisely in the Centre of the concave of the Retina Tunica. And therefore, according to the law of Contrariety, the Cure of this frequent symptome is chiefly, if not only to be hoped from the use of Convex Spectacles, which determine the point of Concurse exactly in the Centre of the Retina Tunica; the rayes, by reason of the double Convexity, viz. of the Lens and Chrystalline Humor, being sooner and more vigorously united, in the due place.

The Other, being Contrary to the first, and alwayes Native, commonly

named Purblindness, Physitians define to be Obscuritus visus in cernendis rebus distantibus; in propinquis verò integrum acumen: a Dimness of the sight in the discernment of Objects, unless they be appropinquate to the Eye. The Causes hereof generally are either the too spherical Figure of the Chrystalline Humor; or, in the Ductus Ciliares, or small Filaments of the Aranea Tunica (the proper investment of the Chrystalline) a certain ineptitude to that contraction, requisite to the adduction of the Chrystal∣line inwards towards the retina tunica, which is necessary to the discernment of objects at distance: either of these Causes making the common Base of the Image to be determined in the Vitrious Humor, and consequently the Image to arrive at the retina tunica, perturbed and confused. And there∣fore our advice is to all Purblind Persons, that they use Concave Spectacles: for such prolong the point of concurse, untill it be convenient, i. e. to the concave of the retina tunica.

Notes

• * 1.1

  Art. 13. PARADOX. That the pro∣spect of a shil∣ling or object of a small dia∣metre is as great, as the Prospect of the Firmament.
• * 1.2

  Art. 14. Why an ob∣ject appears both greater in Dimensions and more Di∣stinct in parts, near at hand, than far off.
• * 1.3

  Art. 15. Why an o••∣ject, specula∣ted through a Conve•• Le••s▪ appears both greater and more distinct; but through a Concave, ••e••s and more Con∣fused: than when specula∣ted only with the Eye.
• * 1.4

  Art. 16. DIGRESSI∣ON.

  What Figur'd Perspicils are convenient for Old: and what for Purblind persons.