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 ...

SECT. II.

TO dispel these Clouds, that have so long eclipsed the splendor of Epicurus Assertion, of the Incidence of Images Visible into the Eye (for we shall not here dispute, whether he intended the sigillation to be made in that Convex Speculum, the Chrystalline Humour; or that Concave one, the Retina Tunica) and explicate the abstruse nature of Vision: we ask leave to possess you with certain necessary Propositions: We assume therefore,

Assumption the First.

That the superfice of no Visible is so exquisitely smooth, polite, or equal,* 1.1 as not to contain various Inaequalities, i. e. Protuberant and Deprest parts, or certain (Monticuli and Valleculae) small Risings and Fallings: which in some bodies being either larger, or more, are discoverable by the naked intuition of the Eye; and in others, either smaller, or fewer, require the detection of the Microscope.

This is neither Praecarious, nor Conjectural: but warranted by Reason, and autoptical Demonstration. For, if the object assumed be polisht Mar∣ble; since that apparent Tersness in the surface thereof is introduced by the detrition of its grosser inaequalities by Sand, and that Sand is nothing but a multitude of Polyedrical solid Grains, by the acuteness and hardness of their Angles cutting and derasing the more friable particles of the Mar∣ble: it must follow, that each of the grains of Sand must leave an impressi∣on of its edge, and so that the whole superfice must become scarified by innumerable small incisions, variously decussating and intersecting each other. If Steel of a speculary smoothness, such as our com∣mon Chalybeat Mirrours; since the Tersness thereof is artificial, caused by the affriction of Files, which cut only by the acuteness of their teeth, or lineal inaequalities: it is not easie to admit, that they leave no scratches, or exarations on the surface thereof; and where are many Incisions, each whereof must in Latitude respond to the thickness of the Tooth in the File, that made it, there also must

be as many Eminences or small Ridges intercepted among them. And if Glass▪ whose smoothness seems superlative; since it is composed of Sand and Salts, not so perfectly dissolved by liquation, as not to retain various An∣gles: it cannot be unreasonable to inferr, that those remaining points or an∣gular parts must render the Composition in its exteriors full of Asperities. And, as for Autoptical Evidence; that Marble, Steel, and Glass are unequal in their superfice, is undeniable not only from hence, that a good Engyscope, in a convenient light, doth discover innumerable rugosities and Cavities in the most polisht superfice of either: but also from hence, that Spiders and Flyes do ordinarily run up and down perpendicularly on Venice Glass, which they could not do, if there were not in the surface thereof many small Cavities, or Fastnings for the reception of the Uncinulae, or Hooks of their Feet. To which may also be added, the Humectation of Glass by any Li∣quor affused; for, if there were no Fosses and Prominences in the superfice thereof, whereon the Hamous particles of the Liquid might be fastned, it would instantly run off without leaving the least of moisture behind. And hence

Assumption the Second.

* 1.2That as the whole Visible Image doth emane from the whole superfice of the object; so do all the parts thereof emane from all the parts of the Object: i. e. that look how many Atoms are designable in the superfice, from so many points thereof do Atoms exhale, which being contiguously pursued by others and others successively deceding, make continued Rayes, in direct lines tending thitherward, whither the faces of the particles point, from which they are deradiated.

For, insomuch as in the superfice no particle can be so minute to the sense, as, in respect to the Asperity, or Inaequality of its surface, not to have vari∣ous Faces, by which to respect various parts of the Medium: it must inevi∣tably follow, that all the rayes effluxed from an object, do not tend one and the same way, but are variously trajected through the Medium, some upward, others downward, some to the right, others to the left, some obversly or to∣ward, others aversly or fromward, &c. So that there is no region or point of the compass designable, to which some rayes are not direct. And from this branch shoots forth our

Third Assumption.

* 1.3That every visible Image is then most Dense and United, when it is first ab∣duced from the Object: or, that by how much the neerer the visible Species is to the Body, from which it is delibrated, by so much the more Dense and United are the rayes of which it doth consist; and so much the more Rare or Disgre∣gate, by how much the farther it is removed from it. This may be exempli∣fied in lines drawn from the Centre of a Circle to the Circumference; for by how much the farther they run from the Centre, by so much the greater space is intercepted betwixt them: and by how much the larger space is in∣tercepted betwixt them, by so much the greater must their Rarity be, the degrees of Rarity being determinable by the degrees of intercepted space.

Thus also must the rayes of the Visible Image, in their progress mutually recede each from other, and according to the more or less of their Elon∣gation from the point of abduction, become more or less Rare and scattered, into the amplitude of the Medium. However, we deny not the necessity of their innumerable Decussations, and Intersections; in respect to the vari∣ous Faces, and Confrontings of the parts of the superfice, from which they are emitted. And hence we extracted our

Fourth Assumption.* 1.4

That the Visible Image, though really diffused through the space of the me∣dium within the sphear of Projection; is notwithstanding neither total in the total space, nor total in every part thereof, as is supposed in the First Ob∣jection: but so Manifold, as there are parts of the Medium, from which the Object is adspectable.

Here may we introduce a Paradox, which yet doth not want a considerable proportion of Verisimilitude to justifie the sobriety and acuteness of his Wit, that first started it; which is, That of divers men, at the same time, specu∣lating the same object, no one doth behold the same parts thereof, that are be∣held by another: nay more, that no man can see the same parts of an Object,* 1.5 with both eyes at once; nay more, not the same parts with the same eye, if he remove it never so little, because the level of the Visive Axe is varied. This may be verified by a single reflection on the Cause hereof, which is the In∣equality, or Asperity of the superfice of Bodies, seemingly most polite: for, in respect of that, it is of necessity, that various Rayes, proceeding from the various parts thereof, variously convene in the parts of the Medium; and insomuch as each of those rayes doth represent that particle only, from which it was effused, and no other, in their concurse they cannot but represent other and other parts, according to the respective places or regions of the Medium, in which the Eye is posited, that receives them. However, we shall familiarize it by Example. Let two men at once behold a Third, one before, the other behind: and both may be said to behold the same man, but, truly, not the same parts of him; because the eyes of one are obverted to his Anterior, and those of the other to his Posterior parts. Take it yet one note higher. Let the Face of a man be the Object, on which though divers persons gaze at the same time, one on the right a second on the left side, a third confrontingly, a fourth and a fifth obliquely betwixt the other three; and all may be said to have an equal prospect of the face: yet can it not be asserted, that they do all see the same parts thereof, but each a particular part. Whence it may be inferred, that albeit we may allow them all to behold his Fore-head, Eyes, Nose, Cheeks, Mouth, &c. yet can we not allow them all to see the same parts of Forehead, Eyes, Nose, Cheeks, &c. because of their unequal situation, which Causeth that the whole spe∣cies prodient from the face, doth not tend into the whole medium, but in∣to various parts of it, respective to the various faces of the deradiant parts. Moreover, because this praesumed Inaequality is not competent only to the greater parts of the face, such as the Eyes, Nose, Mouth, Chin, &c. but as justly considerable in the very Skin, which hath no designable place, wherein are not many smaller and smaller Eminencies and Depressions, de∣prehensible (if not by the Opticks of the body, yet) by the ac••es of the

Mind: hence is, that having imagined the Eyes of the Five Spectators to move their visive Axes from part to part successively, and as slowly as the shadow of the Gnomon steals over the parts of a Dial, untill they have ranged over the whole face; we may comprehend the necessity, of the discovery of a fresh part by every new aime or levell of each eye, and the baulking of others; as if in Particles of devex Figure, no Particels can be detected a new, but as many of those formerly discerned must be lost, and as many, nay more remain concealed.

* 1.6And this Consideration smoothly ushers in two Consectaries

(1) That to say, one simple species doth replenish the whole Medium, is not, in the strict Dialect of Reason, so proper, as to say, the Medium is possessed by an Aggeries, or Convention of innumerable species: which being divers in respect to the divers parts of the Object, from which they were deradiated, must also be divers in their Existence, and Diffusion through the several parts of the Perspicuum. And yet must they be allowed to constitute but one entire species; and this in respect to their Emanation from one Object: because as the single parts of the species represent the single parts of the object, so doth the whole of the species represent the whole of the Object.

(2) That many, nay Myriads of different Species may be Coexistent in the Common Medium,* 1.7 the Aer; and yet no necessity of the Coexistence of many Bodies in one and the same place; it being as justifiable to affirm, that they reciprocally penetrate each others dimensions, as that the Warp and Woof, or intersecting threads in a Cloth, do mutually penetrate each other: be∣cause the Aer is variously interspersed with Inanities, or small empty Roads, convenient to the inconfused transmission of all those swarms of Rayes, of which the species consist. Have you not frequently observed, when many Candles were burning together in the same room, how, according to the various interposition of opace bodies, various degrees of Shadows and Light have been diffused into the several quarters of the same? and can you give any better reason of those various Intersections and Decussations of the se∣veral Lights, then this; that the rayes of Light streaming from the diverse Flames, are directly and inconfusedly trajected through the several inane Receptaries of the Aer, respective to the position of each Candle, without reciprocal impediment; the rayes of one, that are projected to the right hand, in no wise impeding the passage of those of another, that are projected to the left, in the same sensible part of the Aer. Exactly so do the rayes of divers Species Visible, in their progress through the aer, pass on in direct and uninterrupted lines, without Confusion: and though they may seem to possess the same sensible part of the medium, yet will not reason allow them to possess the same Insensible particles thereof; in regard the distinct transmission of each clearly demonstrateth, that each possesseth a distinct place. Nor doth this their Iuxta-position, or extreme Nearness necessitate their Confusion; since we daily observe that Water and Wine may be so Commixt in a Vial, as therein can be assigned no sensible part, wherein are not some parts of both Liquors: and yet most certain it is, that the particles of Wine possess not the same Invisible Loculaments, or Re∣ceptaries, that are replete with the particles of Water, but others absolute∣ly distinct; because otherwise there would be as much of Water, or Wine alone, in the Vial, as there is of both Water and Wine, which in that Con∣tinent

is impossible. And hereupon we Conclude, that to admit every distinct species to replenish the whole medium; is no less dangerous, then to admit, that each of two Liquors confused doth singly replenish the whole Capacity or the Continent: the parity of reasons justifying the Parallelism.

Assumption the Fifth.

That the visible Image, being trajected through the Pupil,* 1.8 and having suf∣fered its ultimate refraction in that Convex Mirror, the Chrystalline Humor; is received and determined in that principal seat of Vision, (which holds no remote analogy to a Concave Mirror) the Retina Tunica, or Expansion of the Optick Nerve in the bottom of the eye: and therein represents the Object from whence it was deradiated, in all particulars to the life, i. e. with the same Colour, Figure, and Situation of parts, which it really beareth; provided the Distance be not excessive.

The First part of this eminent Proposition, that excellent Ma∣thematician, Christopher Scheinerus, hath so evicted by Physical Reasons, Optical Demonstrations, and singular Experiments; as no truth can seem capable of greater illustration, and less opposition: and therefore the greatest right we can do our selves, or you, in this point, is to remit you to the observant lecture of his whole Third Book, de Fundament. Opticis; which we dare commend with this just Elogie, that it is the most Elaborate and Satisfying investigation of the Principal Seat of Vision, that ever the World was enriched with, and He who shall desire a more accomplisht Discourse on that (formerly) abstruse Theorem, must encoun∣ter the censure of being either scarce Ingenious enough to comprehend, or scarce Ingenuous enough to acknowledge the convincing Energy of the Arguments and Demonstrations therein alledged, for the confirmation of his Thesis, Radij formalitèr visorij nativam sedem esse tunicam re∣tinam.

And the other is sufficiently evincible even from hence;* 1.9 That the Sight, or (if you please) the Interior Faculty doth alwayes judge of t••e adspectable form of an Object, according to the Condition of the Image emanant from it, at least, according as it is repre∣sented by the Image, at the impression thereof on the principal visory part. Which is a position of Eminent Certitude. For, no other Cause can be assigned, why the Visive Faculty doth deprehend and pronounce an object to be of this, or that particular Colour: but only this, that the Image imprest on the Net-work Coat doth repre∣sent it in that particular Colour, and no other. Why, when half of the Object is eclipsed, by some opace body interposed, the eye can speculate, nor the faculty judge of no more then the unobscured half: but only this, that the Image is mutilated, and so consisteth of onely those radii, that are emitted from the un∣obscured half, and consequently can inferr the similitude of no more.

Why an Object, of whatever Colour, appeareth Red, when speculated through Glass of that Tincture: but only because the Image, in its trajecti∣on through that Medium, being infected with redness, retains the same even to its sigillation on the Expansion of the Optick Nerve. Why the sight, in some cases, especially in that of immoderate distance, and when the object is beheld through a Reversing Glass, deprehends the object under a false fi∣gure: but because the Image represents it under that dissimilar figure, ha∣ving either its angles ••etused, by reason of a too long trajection through the Medium, or the situation of its parts inverted, by decussation of its rayes in the Glass.

CONSECTARY the First.

* 1.10Now, it being no less Evident, then Certain, that the Image is the sole cause of the Objects apparence under such or such a determinate Colour, and of this or that determinate Figure: it is of pure Consequence, that the Image must also be the Cause of the Objects appearance in this or that de∣terminate Magnitude; especially since Figure is essenced in the Termina∣tion of Magnitude, according to Euclid. (lib. 1. def. 14.) Figura est, quae sub ali∣quo, vel aliquibus terminis comprehenditur. For, why doth the object ap∣pear to be of great, small, or mean dimensions; if not because the Image arriving at the sentient, is great, small, or mean? Why doth the whole ob∣ject appear greater then a part of it self; unless because the whole Image is greater then a part of it self? To speak more profoundly, and as men not altogether ignorant of the Mysteries in Opticks; demonstrable it is, that the Magnitude of a thing speculated may be commensurated by the propor∣tion of the Image deradiated from it, to the distance of the Common Inter∣section. For as the Diametre of the Image, projected through a perspe∣ctive, or Astronomical Tube, on a sheet of white paper, is in proportion to the Axis of the Pyramid Eversed; so is the diameter of the basis of the Object to the Axis of the Pyramid Direct. And hereby also come we to apprehend the Distance of the Object from the Eye; for having obtained the Latitude of the object, we cannot want the knowledge of its Distance: and by conversion, the knowledge of its distance both assists and facilitates the comprehension of its Magnitude. Which comes not much short of absolute necessity; since as Des Cartes (Dioptrices cap. 6.) hath excellently observed, in these words: Quoniam autem longitudo longius decurrentiam radiorum non exquisite salis ex modo impulsus cognosci potest, praecedens Di∣stantiae scientia hic in auxilium est vocanda. Sic, ex Gr. s•• distantia cognos∣catur esse magna, & Angulus visionis sit parvus; res objecta longius distans judicatur magna: sin verò distantia sciatur esse parva, & angulus Visionis sit magnus; objectum judicatur esse parvum, si verò distantia objecti longi∣us dissiti sit in cognita; nihilcerti de ejus magnitudine decerni potest: if the Distance of an object far removed be unknown, the judgment con∣cerning the magnitude thereof must be uncertain.

CONSECTARY the Second.

Again, insomuch as the Receptary of the Visible Image,* 1.11 is that Con∣cave Mirrour, the Retina tunica (we call it a Concave Mirrour, not only in respect of its Figure and Use, but also in imitation of that grand Master of the Opticks, Alhazen, who (in lib. 1. cap. 2.) saith thus; Et sequitur ex hoc, at corpus sentiens, quod est in Concavo Nervi (retina nimirum) sit aliquantu∣lùm Diaphanum, ut appareant in eo formae lucis & coloris, &c.) Hence is it, that no Image can totally fill that Receptary, unless it be derived from an object of an almost Hemispherical ambite, or Compass; so that the rayes, tending from it to the eye, may bear the form of a Cone, whose Base is the Hemisphere, and point (somewhat retused) the superfice of the Pupil. This perfectly accords to Keplers Canon; Visionem fieri, cum totius Hemi∣spherij mundani, quod est ante oculum, & amplius paulo, idolum statuitur ad album subrufum Retinae cavae superficiei parietem. (in Paralipomen. ad Vitellion. cap. 5. de modo Vision. num. 1.) Not that either He, or we, by the Optical Hemisphere, intend only the Arch of the Firmament; but any Am∣bite whatever, including a variety of things obverted to the open eye, partly directly, partly obliquely, or laterally, and Circumqua{que} in all points about.

And this being conceded, we need not long hunt for a reason, why,* 1.12 when the eye is open, there alwayes is pourtraied in the bottom of the eye some one Total Image; whose various parts may be called the Special Images of the diverse things at once objected. For, as the whole Hemisphere Visive includes the reason of the whole Visible: so do the parts thereof include the reason of the special Visibles, though situate at unequal distance. And, since, the Hemisphere may be, in respect either of its whole, or parts, more Remote, and more Vicine; hence comes it, that no more Rayes arrive at the Eye from the Remote, than the Vicine: because in the Vicine, indeed, are less or fewer bodies, than in the Remote, but yet the Particles, or Faces of the particles of bodies, that are directly obverted to the Pupil, are more. Which certainly is the Cause, why of two bodies, the one Great, the other Small, the Dimensions seem equal; provided the Great be so remote, as to take up no greater a part of the Visive Hemisphere, than the small: because, in that case, the rayes emanant from it, and in direct lines incident into the pupill of the Eye, are no more then those deradiate from the small, and consequently cannot represent more parts thereof, or exhibit it in larger Dimensions. Whereupon we may conclude that the Visive Faculty doth judge of the Magnitude of Objects, by the proportion that the Image of each holds to the amplitude of the Concave of the Retina Tunica: or, that by how much every special Image shall make a greater part of the General Image, that fills the whole Hemisphere Visive, and so possess a greater part of the Concave of the Retina Tunica; by so much the greater doth the Fa∣culty judge the quantity thereof to be: and •• Contra. And, because a thing, when near, doth possess a greater part of the Visive Hemisphere, than when remote: therefore doth the special Image thereof also possess a greater part of the Concave in the Retina Tunica, and so exhibit in greater Dimensions; and it decreaseth, or becometh so much the less, by how much the farther it is abduced from the eye; For it then makes room

for another Image of another thing, that is detected by the abduction of the former, and enters the space of the Hemisphere obverted. And here∣upon may we ground a

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,* 1.13 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?

* 1.14To 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,* 1.15 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,* 1.16 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.

Assumption the Sixth and last.

Since all objects speculated under the same Angle,* 1.17 seem of equal Mag∣nitude (according to that of Scheinerus, sicut oculus rem per se parvam, mag∣nam arbitratur, quia sub magno angulo, refractionis beneficio, illam appre∣hendit: & magnam contrario parvam; fundament. Optic. lib. 2. part. 2. cap. 5.) and are accordingly judged, unless there intervene an Opinion of their unequal Distance, which makes the Spectator praesume, that that Object is in it self the Greater, which is the more Remote, and that the Less, which is the less Remote: therefore, to the appehension and Di∣judication of one of two objects, apparently equal, to be really the greater, is not required a greater Image, than to the apprehension and dijudi∣cation of an object to be really the less; but only an opinion of its greater Distance.

This may receive both Illustration and Confirmation from this easie Ex∣periment. Having placed horizontally, in a valley, a plane Looking Glass, of no more then one foot diametre; you may behold therein, at one intuition the Images of the firmament, of the invironing Hills, and all other things circumsituate, and those holding the same magnitude, as when specu∣lated directly, and with the naked eye: and this only because, though the Image in Dimensions exceed not the Area of the Glass, yet is it such, as that together with the things seen, it doth also exhibit the Di∣stance of each from other. Exactly like a good Landskip, wherein the ingenious Painter doth artificially delude the eye by a proportionate diminution and decurtation of the things praesented, insinuating an opinion of their Distance. And therefore, the Reason, why the Images of many things, as of spacious Fields, embroydered with rowes of Trees, numerous Herds of Cattle, Flocks of Sheep, &c. may at once be received into that narrow window, the Pupill of the eye, of a man standing on an Hill, Tower, or other eminent place, advantageous for prospect: is only this, that to the Speculation of the Hemisphere comprehending all those things, in that determinate magnitude, is required no greater an Image, than to the Speculation of an Hemisphere, whose diametre is commensurable

only by an inch. Since neither more rayes are derived from the one to the Pupil of the Eye, than from the other: nor to the judication of the one to be so much Greater than the other, is ought required, beside an Opinion that one is so much more Distant than the other. And this we conceive a sufficient Demonstration of the Verity of our last Paradox, viz. that the Eye sees as much, when it looks on a shilling, or other object of as small dia∣metre; as when it looks on the greatest Ocean.

Here most opportunely occurs to our Consideration that notorious PROBLEM, Quomodo objecti distantia deprehendatur ab oculo? How the Distance of the Object from the eye is perceived in the act of Vision?

This would Des Cartes have solved (1) By the various Figuration of the Eye.* 1.18 Because in the Conspection of Objects remote, the Pupil of the Eye is expanded circularly, for the admission of more Rayes; and the Chry∣stalline Humor somewhat retracted toward the Retina Tunica, for the De∣termination of the point of Concurse in the same, which otherwise would be somewhat too remote: and on the contrary, in the conspection of ob∣jects vicine, the Pupil is contracted circularly, and the Chrystalline Lens protruded somewhat outwardly, for the contrary respects. (2) By the Di∣stinct, or Confused representation of the object; as also the Fortitude, or Imbecillity of Light illustrating the same. Because things represented con∣fusedly, or illustrated with a weak light alwayes appear Remote: and on the contrary, things praesented distinctly or illustrate with a strong light, seem vicine.

* 1.19But all this we conceive unsatisfactory. (1) Because, unless the varia∣tion of the Figure of the Eye were Gradual, respective to the several de∣grees of distance intercedent betwixt it and the object; it is impossible the sight should judge an object to be at this or that Determinate remotion: and that the variation of the Figure of the Eye is not Gradual respective to the degree of distance, is evident even from hence; that the Pupil of the Eye is as much Expanded, and the Lens of the Chrystalline Humor as much Retracted toward the Retina Tunica, in the conspection of an object situate at one miles distance, as of one at 2, 3, 4, or more miles; there be∣ing a certain Term of the Expansion of the one part, and Retraction of the other. (2) Because though Vision be Distinct, or Confused, both accord∣ing to the more or less illustration of the object by light, and to the greater or less Distance thereof from the Eye; yet doth this Reason hold only in mean,* 1.20 not large distance: since the orbs of the Sun and Moon appear grea∣ter at their rising immediately above the Horizon, that is, when they are more Remote from the Eye, than when they are in the Zenith of their gyre, that is, when they are more Vicine to the Eye; and since all objects illustrate with a weak light, do not appear Remote, nor •• contra, as common observation demonstrateth.

And therefore allowing the Acuteness of Des Cartes Conceit, we think it more safe, because more reasonable to acquiesce in the judgment of the grave Gassendus; who (in Epist. 2. de Apparente Magnitud. solis hu∣milis & sublimis) most profoundly solves the Problem, by desuming the Cause of our apprehending the distance of an Object, in the act of Vision,

from a Comparison of the thing interjacent between the object seen, and the Eye. For, though that Comparation be an act of the Superior Faculty; yet is the connexion thereof to the sense, necessary to the making a right judg∣ment, concerning the Distance of the Visible. And, most certainly, there∣fore do two things at distance seem to be Continued, because they strike the Eye with cohaerent, or contiguous Rayes. Thus doth the top of a Tower, though situate some miles beyond a Hill, yet seem Contiguous to the same, nay to the visible Horizon; and this only because it is speculated by the Mediation of Contiguous Rayes: and the Sun and Moon, both ori∣ent and occident, seem to cohaere to the Horizon because though the spaces are immense, that intercede betwixt their Orbs and the Horizon, yet from those spaces doth not so much as one single Raye arrive at the Eye, and those which come to it from the Sun and Moon are contiguous to those which come from the Horizon. And hence is it, that the Tower, Hill, and Hori∣zon seem to the sight to be equidistant from the Eye; because no other things are interposed, at least, seen interposed, by the comparison of which, the one may be deprehended more than the other. Besides, the distance of the Horizon it self is not apprehended by any other reason, but the di∣versity of things interjacent betwixt it and the Eye: for, look how much of Space is possessed valleys and lower grounds interjacent, so much of Space is defalcated from the distance; the sight apprehending all those things to be Contiguous, or Continued, whose Rayes are received into the Eye, as Contiguous, or Continued, none of the spaces interjacent affording one raye. Of which truth Des Cartes seems to have had a glimpse, when (in Dioptrices cap. 6. Sect. 15.) he conceds; objectorum, quae intuemur, prae∣cedaneam cognitionem, ipsorum distantiae melius dignoscendae inservire: that a certain praecognition of the object doth much conduce to the more certain dignotion of its Distance.

And on this branch may we ingraft a PARADOX;* 1.21 that one and the same object, speculated by the same man, in the same degree of light, doth al∣wayes appear greater to one Eye, than to the other. The truth of this is evin∣cible by the joint testimony of those incorruptible Witnesses of Certitude, Experience and Reason. (1) Of Experience, because no man can make the vision of both his eyes equally perfect; but beholding a thing first with one eye, the other being closed, or eclipsed, and then with the other, the former being closed or eclipsed; shall constantly discover it to be greater in dimensions in the apprehension of one Eye, than of the other: and Gassendus, making a perfect and strict Experiment hereof, testifies of himself, (in Epist. 2. de Apparent. Magnitud. Solis, &c. Sect. 17.) that the Characters of his Book appeared to his right Eye, by a fifth part, greater in dimensions, though somewhat more obscure, than to his left. (2) Of Reason; because of all Twin Parts in the body, as Ears, Hands, Leggs, Testicles, &c. one is alwayes more vigorous and perfect, in the performance of its action, than the other. Which Inaequality of Vigour, if it be not the Bastard of Custom, may rightfully be Fathered upon either this; that one part is invigorated with a more liberal afflux of Spirits, than the other: or this, that the Orga∣ganical Constitution of one Part is more perfect and firm, than that of the other. And, therefore, one Eye having its Pupill wider; or the figure of the Chrystalline more Convex, or the Retina Tunica more concave, than the other; must apprehend an object to be either larger in Dimensions, or more Distinct in Parts, than the other, whose parts are of a different confi∣guration:

either of these Causes necessitating a respective Disparity in the Action.

* 1.22If this sound strange in the ears of any man, how will he startle at the mention of that much more Paradoxical Thesis of Ioh. Baptista Porta (lib. 6. de Refra••tion. cap. 1.) That no man can see (distinctly) but with one eye at once? Which though seemingly repugnant not only to common per∣suasion, but also to that high and mighty Axiom of Alhazen, Vitellio, Franc. Bacon. Niceron, and other the most eminent Professors of the Optiques, That the Visive Axes of both eyes concurr and unite in the object speculated: is yet a verity, well worthy our admission, and assertion. For, the Axes of the Eyes are so ordained by Nature, that when one is intended, the other is relaxed, when one is im∣ployed, the other is idle and unconcerned; nor can they be both in∣tended at once, or imployed, though both may be at once relaxed, or unimployed: as is Experimented, when with both eyes open we look on the leaf of a Book; for we then perceive the lines and print thereof, but do not distinctly discern the Characters, so as to read one word, till we fix the Axe of one eye thereon; and at that instant we feel a certain suddain sub∣sultation, or gentle impulse in the Centre of that eye, arising doubtless from the rushing in of more spirits through the Optick Nerve, for the more efficacious performance of its action. The Cause of the impossibility of the intention of both Visive Axes at one object, may be desumed from the Parallelism of the Motion of the Eyes; which being most evident to sense, gives us just ground to admire, how so many subtle Mathematicians, and exquisite Oculists have not discovered the Coition and Union of the Visive Axes in the object speculated, which they so confidently build upon, to be an absolute Impossibility. For, though man hath two Eyes; yet doth he use but one at once, in the case of Distinct inspection, the right eye to dis∣cern objects on the right side, and the left to view objects on the left: nor is there more necessity, why he should use both Eyes at once, than both Arms, or Leggs, or Testicles, at once. And for an Experiment to assist this Reason; we shall desire you only to look at the top of your own Nose, and you shall soon be convicted, that you cannot discern it with both eyes at once; but the right side with the right eye, and afterward the left side with the left eye: and at the instant of changing the Axe of the first eye, you shall be sensible of that impulse of Spirits, newly mentioned. No••, indeed, is it possible, that while your right eye is levelled at the right side of your nose, your left should be levelled at the left side, but on the contrary averted quite ••rom it: because, the motion of the eyes being Conjugate, or Parallel, when the Axe of the right eye is converted to the right side of the nose, the Axe of the left must be converted toward the left Ear. And, therefore, since the Visive Axes of both Eyes cannot Concurr and Unite in the Tipp of the Nose; what can remain to persuade, that they must Concurr and unite in the same Letter, or Word in a book, which is not ma∣ny inches more remote than the Nose? And, that you may satisfie your self, that the Visive Axes doe never meet, but run on in a perpetual Paral∣lelism, i. e. in direct lines, as far distant each from other, as are the Eyes themselves; having fixed a staff or launce upright in the ground, and retreat∣ed from it to the distance of 10 or 20 paces, more or less: look as earnestly as you can, on it, with your right eye, closing your left, and you shall per∣ceive it to eclipse a certain part of the wall, tree, or other body situate beyond it. Then look on it again with your left eye, closing your right; and you

shall observe it to eclipse another part of the wall: that space being inter∣cepted, which is called the Parallaxe. This done, look on it with both eyes open; and if the Axes of both did meet and unite in the staff, as is gene∣rally supposed, then of necessity would you observe the staff to eclipse ei∣ther both parts of the Wall together, or the middle of the Parallaxe: but you shall observe it to do neither, for the middle shall never be eclipsed; but only one of the parts, and that on which you shall fix one of your eyes more intently than the other. This considered, we dare second Gassendus in his promise to Gunners, that they shall shoot as right with both eyes open, as only with one: for levelling the mouth of the Peece directly at the mark, with one eye, their other must be wholly unconcerned there∣in, nor is it ought but the tyrannie of Custome, that can make it difficult.

Here, to prevent the most formidable Exception,* 1.23 that lyes against this Paradox, we are to advertise you of two Considerables. First, that as well Philosophers, as Oculists unanimously admit three Degrees, or gradual Differences of sight. (1) Visus Perfectissimus, when we see the smallest (visible) particles of an object, most distinctly: (2) Perfectus, when we see an object distinctly enough, in the whole or parts, but apprehend not the particles, or minima visibilia thereof: (3) Imperfectus, when besides the object directly obverted to the Pupil of the eye, we also have a glimmering and imperfect perception of other things placed ad latera, on the right and left side of it. Secondly, that the verity of this Paradox, that we see but with one eye at once, is restrained only to the First and Second degrees of Sight, and extends not to the Last. For, Experience assures, that, as many things circumvicine to the principal object, on which we look only with one eye open, praesent themselves together with it, in a confused and obscure manner: so likewise, when both eyes are open, many things, obliquely in∣cident into each eye, are confusedly, and indistinctly apprehended. So that in confused and Imperfect Vision, it may be truly said, that a man doth see with both eyes at once: but not in Distinct and Perfect.

Notes

• * 1.1

  Art. 1. That the su∣perfice of no¦body is per∣fectly smooth: evicted by so∣lid Reas••n, and Aut••psie.
• * 1.2

  Art. 2. That the visi∣ble Image doth consist of so many Rayes as there are Points design∣able in the whole super∣fice of the ob∣ject: and that each Ray hath its line of Tendency di∣rect, respective to the face of that particle in the super∣fice, from which it is e∣mitted.
• * 1.3

  Art. 3. That the Den∣sity and Vnion of the Rayes, composing the visible Image, is greater or less; accord∣ing to their less, or greater Elongation from the Ob∣ject.
• * 1.4

  Art. 4. That the Visi∣ble Image is neither total in the total medium; nor total in every part thereof: but so manifold as are the parts of the medium from which the ob∣j••ct is discern∣able. Contra∣ry to the Ari∣stoteleans.
• * 1.5

  Art. 5. PARADOX▪

  That no man can see the same particle of an object, with both Eyes at once; nay, not with the same Eye, if the level of its Visive Axe be changed
• * 1.6

  Art. 6. CONSECTA∣RY.

  That the Me∣dium is not possessed with one simple I∣mage; but by an Aggregate of innumera∣ble Images, de∣radiate from the same ob∣ject: all which notwithstand∣ing constitute but one entire Image.
• * 1.7

  Art. 7. CONSECTA∣RY 2.

  That Myriads of different Images, ema∣nant from dif∣ferent objects, may be Coex∣istent in the Aer; without reciprocal pe∣netration of Di∣mensions, or Confusi••n of particles: con∣trary to the Pe••ipateticks.
• * 1.8

  Art. 8. That the place of the visible Images ultimate Re∣ception, and complete Per∣ception; is the Concave of the Retina Tu∣nica.
• * 1.9

  Art. 9. That the Fa∣culty forms a judgm••nt of the Conditions of the Object, ac∣cording to the representation thereof by the Image, at its impression on the principal part of Vision, the Amphible∣str••ides.
• * 1.10

  Art. 10. CONSECTA∣RY.

  That the I∣mage is the Cause of Ob∣jects apparence of this or that determinate Magnitude.
• * 1.11

  Art. 11. CONSECTA∣RY 2.

  That no I∣mage can re∣p••enish the C••ncave of the Retina Tuni∣ca, unless it be deradiated from an object of an almost Hemispherical ambite.
• * 1.12

  Art. 12. Why, when the Eye is o∣pen there is a••∣wayes pour∣trayed in the bottom there∣of, some one T••tal Image; whose vario•••• Parts, are the Special ••mages of the several things inclu∣ded in the vi∣sual Hemi∣sphere.
• * 1.13

  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.
• * 1.14

  Art. 14. Why an ob∣ject appears both greater in Dimensions and more Di∣stinct in parts, near at hand, than far off.
• * 1.15

  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.
• * 1.16

  Art. 16. DIGRESSI∣ON.

  What Figur'd Perspicils are convenient for Old: and what for Purblind persons.
• * 1.17

  Art. 17. That to the Di••udication of one of two ob••ects, appa∣rently Equal, to be really the Greater; is not required a greater I∣mage: but on∣ly an Opinion of its greater Distance.
• * 1.18

  Art. 18. Des Cartes O∣pinion, con∣cerning the Reason of the Sights appre∣hending the Distance of an object:
• * 1.19

  Art. 19. Unsatisfacto∣ry; and that for two Con∣siderations.
• * 1.20

  Art. 20. And that more solid one of Gassendus (viz. that the Cause of our apprehen∣ding the Di∣stance ••f an ob∣ject, consisteth in the C••mpara∣tion ••f the 〈◊〉〈◊〉 things 〈◊〉〈◊〉 ••••wixt the o••ject and the 〈◊〉〈◊〉, by the Rati••nal Facu••∣••••) embraced and corrobo∣••••ted
• * 1.21

  Art. 21. PARADOX. That the same Object, specu∣lated by the same man, at the same di∣stance, and in the same de∣gree of light; doth alwayes appear greater to one Eye, than the other
• * 1.22

  Art. 22. A ••econd PARADOX. That all men see (distinct∣ly) but with ••ne Eye at once: contra∣ry to that e∣minent Opti∣cal Axiom▪ that the V••sive Axes of both eyes concurr and unite in th•• o••∣j••ct.
• * 1.23

  Art. 23. The three De∣grees of Visi∣on, viz. most perfect, perfect, and imperfect: and the verity of the Para∣dox restrained only to the two former De∣grees.