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 ...

Assumption the Sixth and last.

Since all objects speculated under the same Angle,* 1.1 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.* 1.2 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.

* 1.3But 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,* 1.4 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;* 1.5 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.

* 1.6If 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,* 1.7 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. 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.
• * 1.2

  Art. 18. Des Cartes O∣pinion, con∣cerning the Reason of the Sights appre∣hending the Distance of an object:
• * 1.3

  Art. 19. Unsatisfacto∣ry; and that for two Con∣siderations.
• * 1.4

  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
• * 1.5

  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
• * 1.6

  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.
• * 1.7

  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.