An entire body of philosophy according to the principles of the famous Renate Des Cartes in three books, (I) the institution ... (II) the history of nature ... (III) a dissertation of the want of sense and knowledge in brute animals ... / written originally in Latin by the learned Anthony Le Grand ; now carefully translated from the last corrections, alterations, and large additions of the author, never yet published ... by Richard Blome.

CHAP. XIX. How Vision, or the Sense ef Seeing is per∣formed.

I. The Soul feels or is sensible by means of the motion of the Nerves. HAving thus explained how the Light is transmitted through the Humours of the Eye; how Objects communicate their Images to the Organ, and how the said Images enter the Brain; it remains now that we explain, how this Image, communicated to the Brain, doth produce that Sensation in us, whereby we are said to See; and in the next place what are the Causes of the Clarity and distinction of our Sight; and Lastly, how the Qualities of Objects, viz. their Situati∣on, Distance, Magnitude, Figure, Motion or Rest, are thereby discerned. Now that we may the better understand how this Spiritual Image is de∣lineated in us, we are to call to mind, what hath been before handled Chap. X. § 6. viz. that such is the Nature of our Soul, that by the force of those Motions, which are imprest on that part of the Brain, whence the thin Capillaments, or Hair∣like Strings of the Retina derive their Original, various Sensations are excited; so that the whole difference of them depends on the various motion, which every part of the Object doth excite. Thus the Soul, by occasion of some motions that are made in the Nerves which belong to the Ears, perceives Sounds; and by means of the motion of those Nerves, that assist the Organ of Tasting, and act immediately upon our Soul, the Sense of Taste is stirr'd up in us, and so of the rest.

II. What the Images of Objects are. For these Impressions are nothing else, but va∣rious motions of the parts of the Brain, and of the Animal Spirits, affecting the Glandula Pinealis or Pine-Apple-like Kernel; to which when the Soul of Man attends, he discerns visible Objects, tho▪ these motions are not like the Objects which they represent. Just after the same manner as one that walks in the Dark, or i•• blind, distinguisheth Ob∣jects by means of a Stick, tho' neither the Stick, nor its Motion bear the Image of them. Or as Words written or spoken, serve to convey the knowledge of things to us, tho' they be no re∣semblances of the things they bring to our Minds.

III. How it comes to pass that some Objects appear more clear∣ly than others. It is manifest therefore, that the sight of any thing will be more clear and lively, according as there are more Rays proceeding from the Object, and entring the Eye: For by this means the im∣pression made upon the Capillaments of the Optick Nerve is the stronger. The largeness of the Ap∣ple of the Eye, conduceth also to this purpose, by giving way to many Rays, proceeding from the same point of the Object, to enter the Eye, in or∣der to the representing of its Image in the bottom thereof. Hence it is that we dilate and open the Apple of our Eye more in viewing a distant Ob∣ject, than one that is near us; because then more Rays enter the Eye from the several points of it, than when we do straiten it and make it less. And for this reason it is that remote Objects, appear more clearly to us, than such as are very near to us, and the Colours of those appear more lively, but of these more dull and weak.

IV. What is the Cause of the Di∣stinction of Vision. As to the Distinction of Sight, whereby the parts of the Object are discerned in their proper Place, Situation, Figure and Colour, it is certain that the same proceeds from the Refraction of Rays. Now to the end that the Sight of any Object may be very distinct, and admit of no confusion at all, it is necessary that all the Rays, which from the same point of the Object, are directed to the same point of the Horny Membran, be so refracted, as that they may at last meet and be united in the point of the bottom of the Eye. But seeing that this doth not happen, save only in those Rays, which proceed from that point of the Object, in which the Axis of Vision is terminated, it so happens that at that time, we can only have a distinct Sen∣sation of that part, and of the rest a confused and indistinct view.

V. The other Cause of Distinct Seeing. The other cause of Distinct Vision, depends on the Capillaments of the Optick Nerve; for seeing that we cannot discern the parts of Bodies we do behold, but only inasmuch as they are distinguisht by Colour, and that the distinct perception of these Bodies, doth not only depend upon this, that all the Rays which proceed from the several points of Bodies, do in the bottom of the Eye meet in about so many other points; but is also caused by the multitude of the Filaments of the Optick Nerve, the Extremities or Ends whereof are contained in that space, which the Image at the bottom of the

Eye doth take up; it follows that there must be so many ends of these Capillaments, as there are sen∣sible parts in the Object, that reflect their Rays. For if the Rays proceeding from 2 different parts of the Object, should meet in 2 divers points of the same Filament separately, it would be the same as if they met in the same Point, because they could not move that one Filament after 2 several manners. Whence it is that Fields, in the Sum∣mer-time, abounding with Red or Yellow Flowers that grow amongst the Grass, do seem to be wholly Red or Yellow; because both the Grass and Flowers acting together upon one and the fame Capilla∣ment, the Flowers which are of a more lively Co∣lour, do only appear to the Eye, because that Fi∣lament, at that time only follows the motion, which the Flowers imprest upon it.

VI. How the Situation of the Ob∣ject comes to be dis∣cerned. The situation of Bodies is not perceived by any Representation or other Action, proceeding from them, but only by the Impulse, coming from a certain Region or quarter, to some particular part of the Brain. For the Object seems to be situate in that part, from whence the Rays come that af∣fect the Eye. Thus we suppose the Candle AC, to be placed in such a quarter, because the Eye BDE, doth from thence receive the Rays that make an Impression upon it, which Impression gives * 1.1occasion to the Soul to judge that the Candle AC, takes up such a situation amongst other Bodies. For the knowledge of the situation of any thing depends only on the situation of the particles of the Brain, from whence the Filaments of the Nerves do arise: But forasmuch as those Filaments are diversly affected by Bodies in different places, because of their various disposition, the Soul by their peculiar motion, discerns their different situa∣tions. Thus when we turn our Head or Eye this way or that way, the Soul is put in mind of that thing, by the Change which the Nerves inserted in the Muscles subservient to that Motion, do ef∣fect in our Brain. For when the Eye BDE sees the Candle AC, the Soul may know the situation of the said Candle, because the Nerves of the Eye partake of another disposition, than if it lookt ano∣ther way.

VII. Why the Image of an Object is turn'd upside down in the Eye. Nevertheless it is to observed, that tho' Objects be perceived in their natural situation; yet their Images are turned upside down in the Eye. The reason whereof is, because but one of those Rays that enter the Eye being Perpendicular, and Direct, and the rest, because of the straitness of the Ap∣ple of the Eye entring obliquely, it happens that the Oblique Ray AB, proceeding from the upper part of the Candle, and the Ray, CD, flowing from its lower, do intercept one another, so that the lower part C, of the Candle, is represented at the bottom of the Eye upwards about D, and the upper part A, of the Candle appears about B, by which means the Candle is delineated in the Eye inverted. This any one can make an Experiment of, by taking the Eye of an Animal, devested of all its Membrans and Muscles besides the Retina; for supposing that this Eye be placed before the little hole of a Darkened Room, and the Candle AC, be set opposit to it, if then standing in the Room we look into the bottom of the Eye, we shall see the Image turn'd upside down.

VIII. Things at a great di∣stance ap∣pear higher This inversion of the Image in the Eye, is th•• cause why things appear by so much higher as they are farther from us; because Rays that pro∣ceed from things near to us, do reach the upper parts of the Eye or Retina; whereas those that flow from Objects far distant, do come to the lower part of the Eye: And consequently it is ne∣cessary, that the former should appear Higher, and these latter Lower.

IX. How the Distance of things is perceived. The Distance of Objects is discerned by the va∣rious motions that change the Figure of the Eye: For when we behold things at a Distance, the Ap∣ple of the Eye becomes more dilated, and the Cry∣stalline Humour is somewhat withdrawn towards the Retina, and by this means the Figure of the Eye becomes more Round: whereas, when we view Objects that are near to us, the opening of the Apple of the Eye is lessened, and the Crystalline Humour contracted, and thrust outwards, by which means the Eye becomes more extended in length. And whilst we thus change the Figure of our Eye, according to the various situation of the Object, an alteration consequently happens in the parts of the Brain, Nature having so ordered it, that the Soul thereby might be informed of the distance of the Object.

X. Other ways of discern∣ing the Di∣stance of Objects. The Distance also of Objects may be discerned by the distinct or confused Representation of them, and so likewise by the strength or weakness of the Light. Thus when we know the Bulk of a Body beforehand, its distinct Figure, and the Liveliness of its Colour, this knowledge will help us to know the distance of it. The Interposition also of many Bodies between us, and those Bodies which we be∣hold, conduceth not a little to the perceiving of their Distance; because the distance we imagin there is between them, serves us for a measure whereby to take the distance of the Objects we are viewing. Thus when the Moon is very high raised above the surface of the Earth, it seems nearer to us when no visible Bodies interpose between us and it, than when many Terrestrial Bodies intervene. The strength also and weakness of the Light are of use to inform us of the Distance of Objects: For Bodies illustrated with a weak Light, are appre∣hended to be at a greater distance, and those things to be nearer to us which are seen distinctly, and under a strong and vigorous collustration. And therefore it is that towards Night, or in misty Wea∣ther, things that are near appear as if they were at a distance from us.

XI. How the Bulk of the Object is perceived. By perceiving the Situation and Distance of eve∣ry Object, we are informed of the Bulk and Big∣ness of it: Thus when the Rays from A and C, are decussated, or intersect one another at the Ap∣ple of the Eye E, the Objects Angle of Vision be∣ing known, the Soul by this Impression, being in∣formed of the length of these Rays, easily discerns the Quantity of the line AC, which is the Magni∣tude of the thing. So that if ever the Mind mi∣stake in judging of the Bulk of any thing, it is on∣ly because it hath not rightly perceived the distance of it. As it happens to a person, who not being able to conceive the great distance there is between the Sun and the Earth, will never know the true Mag∣nitude of the Sun. And this is the Reason why the Sun and Moon, when they are nearest to the Horizon, appear bigger than when they are farther from it; which doth not happen because they sometime appear under a greater, and at other time under a less Angle, but because they are judged to

be at a greater distance. For our Modern Astrono∣mers, who have measur'd the Angle under which the Sun or Moon appear at their Rising and Setting, find it to be the same with that under which it appears about the Meridian.

XII. How the Figure of Objects is perceived. We judge of the figure of any Object, from the knowledge or opinion we have of the situation of the different Parts of it, and not at all from the likeness of the Images, that are delineated in the Eye; forasmuch as those Images are often Elliptical, and of an oblong Square, which represent to us Objects that are perfectly Round and Equilateral Squares. Thus Square Objects, that are at a great distance from us appear Round, because the great distance of the Angles obliterates the Jettings-out and Unevenness of the Object. In like man∣ner, Round or Concave Bodies appear with flat and even Surfaces, because the rising or depression of any of the Parts, by reason of the great distance, escapes the Eye. Thus he who beholds the Sun, doth not discern it to be round, but flat, tho' indeed the middle parts thereof be nearer to our Eyes, than those that are towards the Edges: but so vast is the distance between the Sun and our Eye, that the foresaid proximity of the middle Parts, is not at all considerable or percepti∣ble.

XIII. How the Motion or Rest of Bodies is perceived. Lastly, Motion is perceived when the Images of Objects imprest on the Eye, do run through several Spaces in the Retina, and successively an∣swer to other Images of Objects, which we look upon as immoveable. Or when the Eye is fain to turn it self, that it may continue to have the Sight of the Object. So likewise the Rest of Visible things is perceived, when the Eye continuing with∣out Motion, the Images represented in the Retina are at Rest, and continue to have the same Re∣spect to another Object, which is consider'd as with∣out motion.

XIV. There must be a due Distance betwixt the Object and the Eye. Moreover it is to be observed, that in order to the due and right seeing of any Object, there is required a due distance, to the end that the many Rays that proceed from the several Points of the Object, may meet together in every least part of the Retina. As likewise, that the Image of the Object, may be represented in the bottom of the Eye sufficiently great and perceptible. Otherwise if the Object, for Example D, be too near, the Rays that proceed from it, as ABC, will enter so ob∣liquely into the Convex Surface of the Eye AC, * 1.2that the Retina E cannot be affected by them, as not being sufficiently gathered together. Where∣fore since the meeting of the Rays is only in F, it is no wonder if no perceptible Image of the Object be delineated in the bottom of the Eye. Forasmuch as the Action of Seeing cannot be per∣formed, except the Rays entring the Apple of the Eye, and being refracted at their meeting with the Membrans and Humours, strike upon the Reti∣na, and impress the Image of the Object upon it.

XV. The too great di∣stance of the Object produceth only an obscure Vision. But when the Object is at too great a distance, the contrary happens; for seeing that then only a few Rays proceed from every Point of the Object, and that those that come from them, do too much approach to a Parallelism, it so happens that they are united before they reach the Retina; and pre∣sently after separating themselves again from that Point of their meeting, cannot sufficiently move or agitate the Retina; whereupon only an obscure Image of the Object is represented. By Example, If the Object A be supposed to be far distant from the Organ, it is evident that only a few Rays will flow from each Point of it; and that the Rays * 1.3AB, AC, AD, proceeding from each Point of it, by their inclining too much to a Parallelism, will unite about E, before they come to the Re∣tina F. Whence it will follow, that either a too little Image of the Object will be drawn at the bottom of the Eye, or none at all.

XVI. VVhat a Telescope or Prospe∣ctive-Glass is, and how Objects at a distance are percei∣ved by it. To prevent this defect, a Telescope or Prospe∣ctive Glass is employ'd (that is, an Instrument whereby the Visive Faculty is assisted to discern Objects, that are at a great distance distinctly) which consists of 2 Glasses, viz. a Convex and a Concave: For by the first of these the Rays are so gather'd, that before the union or coition of Cones like to those which are produced by the Eye, the Concave intervenes, which by somewhat dilating of these Cones, doth advance their Points or Cusps, and renders them more distinct; so that being received into the Apple of the Eye after decussation, they again meet in the Retina, and according to the proportion of the Convexity, represent the thing greater. Wherefore, if such an Instrument be duly applied to the Eye, it makes the Rays that are gather'd about E, and afterwards scatter'd again in the Retina F, become gather'd in it, by means of the convexity of the Eye, and so produce a distinct and exact Vision or Sight. For the Telescope, by reason of its great Pupil, makes the Object to be seen by many Rays, and because of the Convex figure of the outward Glass, which congregates the Rays, it appears under a greater Angle of Vision. This Instrument causeth also a great decussation of the Rays, which enter the Round outward Extremity of the Tube, towards the Retina; and by this means a great Representation or Image of the Object, is deline∣ated in the Eye, so that things at a distance appear great and distinct.

XVII. Of the Deception of the Sight. How liable we are to be deceived by our Sight, as well as by our other Senses, will be evident to us by considering the manner how all our Sensa∣tions are formed, and more particularly that of Seeing. For hence it is, that some Objects appear more clearly to us than others; and that their Place, Situation, Figure, and Colours, are not always distinctly represented; that their Bulk seems sometimes greater, and at other times less; yea, and sometimes they are altogether invisible, espe∣cially those Parts that make their Essential constitution. In the Bud of a Tulip-Root, with a Microscope, we may easily discover the Leaves, which afterwards turn Green, those which are to compose the Flower it self, and that little Triangular part which contains the Seed, with the 6 little Pillars that surround it, at the bottom of the Tulip. And the same may be said of the Bud of a Mustard-Seed, of the Kernel of an Apple, and generally of all sorts of Trees and Plants: For tho' nothing of all this can be per∣ceived by the Eyes, no not when assisted by a Microscope; yet we may with confidence conclude that they are all contain'd in the Bud of their Seeds. This may also appear in little Animals, as in the Mites that breed in rotten Cheese, and those little Worms that gnaw the Skin, and cause

the hands to itch, which have all of them Orga∣nical Bodies, as well as any other Animals. And as we see in the Bud of a Root of the Tulip, the whole Flower, so likewise we may perceive in the Treadle of an Egg, which the Hen hath not yet sit upon, a Pullet, which it may be is wholly formed. So likewise Frogs may be perceiv'd in the Spawn of Frogs. To which add what Malbranche asserts, that probably there are infinite Trees in one only Bud, and that all the Plants, and all the Bodies of Man and Animals that shall ever be, have been produc'd from the beginning of the World. Now it is certain, that all these things cannot be perceived by our unassisted Eyes; yea, and that the greatest part of them cannot be per∣ceived with the help of the most excellent Micro∣scopes: And much less by our other Senses, which the Sight excells in order, dignity and extent. Neither must this seem any whit strange unto us, forasmuch as it is only the Surface of Bodies that affects our Senses, and that it is not their whole Surface neither that is capable of affecting our Senses, but those Parts only of it that are big enough to move the Fibres of the Nerves. And forasmuch as those Parts which make up the Essen∣tial Constitution of the Body, are very often too small to move the Fibres of the Nerves, that be∣long to the Organs of Sensation; it must follow that they are hid from us, and that the Object which we perceive, may be quite different from what it appears to be.

Notes

• * 1.1

  Figure 72.
• * 1.2

  Figure 73.
• * 1.3

  Figure 74