The Answer of Monsieur Mariotte to Monsieur Pecquet, about the Opinion, That the Choroeides is the Principal Organ of Sight; Communicated to the Publisher from Paris, and Englished as Follows

The Answer of Monsieur Mariotte to Monsieur Pecquet, concerning the Principal Organ of Vision; where occur divers considerable Experiments. A Continuation of Dr Edward Brown's Account concerning the Mines of Hungary; where is a particular relation of the Hungarian Copper-mine, together with a Narrative of the many natural Baths in that Country and in Austria. An Account of 3 Books: I. COSMOPOEIA DIVINA, per Ludov. de Beaufort M.D. II. CAR-TESIUS MOSAIZANS, Auth. Joh. Amerpoel. III. The DIVINE HISTORY of the GENESIS of the WORLD, explicated and illustrated. The Answer Of Monsieur Mariotte to Monsieur Pecquet, about the Opinion, that the Choroedes is the Principal Organ of Sight; communicated to the Publisher from Paris, and Englished as follows. SIR, I have in your Answer * seen the reasons, which hinder you from believing that the Choroedes is the Principal Organ of Sight; but though they were very Acute, and carried a great deal of Probability with them, yet I did not find them strong enough enough to oblige me, to grant back again this Preeminence to the Retina. You say, in your first Objection, that if the Sclerotis and the Choroides be taken away from an Eye that is very fresh, and that the Retina be left distended on the Vitreous Humor, one shall not be able to see well through this membrane: whence you conclude, that it is not transparent enough, to let so much light, as is sufficient for vision pass upon the Choroides. In my opinion, there is good reason to doubt of this consequence, seeing there may be a great deal of difference between the Retina of a dead Animal, after it hath been exposed to the Ayre, and that of a living Animal, while it is exactly shut up between the Vitreous humour and the Choroides. Different dispositions do ordinarily change the qualities of things; Far, which is transparent when it is melted, grows opacous after it is cold again; and the Tunica cornea of an Eye being held some hours in ones hand, in a hot Ayre grows thick, and a little after entirely opacous. But to the end you may be perswaded, that the Choroides is sufficiently enlightened in a living Animal, you must take the Eye of an Ox (newly kill'd) while it is hot, and cut it in two, in such manner that a good part of the vitreous humour may remain extended upon the Retina; then you shall see distinctly the colours of the Choroides, the basis of the Optick nerve, the trunk of the little vessels which proceed from thence, and their dispersion through the thickness of the Retina, with so much perspicuity that you cannot even discern, whether there be a Retina beyond the Vitreous humour or no. Hence you may judge, that the light, which the Objects send to the Choroides, is more than sufficient for vision, seeing that being much weakened by the reflexion and by a second passage through the Retina and the Vitreous humour, it is yet strong enough to form a clear and distinct vision of the Choroides in our Eyes. Nor that I deny the Retina to have some whiteness in a living Animal, and that it is a little less transparent than the other Humours, chiefly in that part which is contiguous to the Choroides; and Nature may have made it so on purpose to temper the brightness of great lights, and to hinder dazelings, like like manner as she hath spread over our skin an insensible Epidermis, to keep it from being too easily hurt by those bodies that touch us, and by the Excess of heat and cold. But if I should absolutely deny, that the Retina hath any opacity in a living Animal, your Experiment would not convince me, it being made but upon a Retina, whose more subtle and transparent parts are evaporated; and I might propose to you for an example a piece of white paper, through which, when it is wet, one sees distinctly enough whatever is contiguous to it; and which presently resumes its first opacity, when it is a little while exposed to the Air: And if this Example were not sufficient, I might alledge the little Chrystalline, which is found in the middle of the Chrystalline humour of many Animals, and is as it were the kernel of it; the which, being as transparent as the other humours of the Eye in a living Animal, becomes in two or three days after his death white and opacous, notwithstanding that it be left inclosed in the Eye, and that the Exterior Chrystalline remain still transparent. Your second Experiment to prove the opacity of the Retina, which is to put it into water, is also extremely deceitful: For, you make no doubt but the Hyaloecides, which envelopes the Vitreous humour, is perfectly transparent: nevertheless if you lay in a dish half full of water, part of the Vitreous humour, the parts of the Hyaloecides, which stick to it, will appear whitish and thick like a spiders web, although the Vitreous humour do still retain its transparency. It is not then a sufficient proof to know, whether the Retina be opaque in a living Animal, by putting it into the water: And by what tryal soever you make of it, after it has been exposed to the Ayre, you can draw no consequence to prove that it is opaque in its Natural state: For the Chrystalline itself becomes a little thickish in water, and if it be left there some time, or exposed to the frost, it becomes white and opaque like snow. It is therefore necessary for resolving our difference, and for knowing with certainty, whether the light of objects pass almost all entire to the Choroeides, or whether it is almost all intercepted by the Retina, to bring observations made on the Retina and the Choroeides, while they are in their natural state, as I shall do in the following Experiment. Place by night a lighted Candle very near your Eyes, and cause a dog, distant from the candle 8 or 10 paces, to look upon you, then you shall see in his eyes, a light sufficiently bright, which I hold to proceed from the reflexion of the light of the Candle, whose image is painted on the Choroeides of the dog, which having much whiteness and luster causes this very strong reflexion; for if it proceeded from the Chrystalline or Retina, the same appearance would be seen in the Eyes of Men, Birds, and other animals, who have the Choroeides black, which is not found so by us. It is therefore manifest by this Experiment, that the luminous rays do pass with a great deal of force as far as the Choroeides, and that the Retina receives very little impression. Now this appearance is made in this manner: The little picture of the candle which is upon the Choroeides of the dog, (where is the Focus of the Chrystalline and other humours together) sending rays back through these humours, makes its reciprocal Focus towards the candle, and by consequence the eyes which are near to the point, where these rays do reunite, ought to see the Chrystalline of the dog very much illuminated. They that are skilled in Opticks know the demonstration: and there is an effect, wholly like unto this, to be seen by a very easy Experiment. Place around Glass-bottle, full of very clear water, 8 or 10 paces from a Candle, and put behind the bottle, at near the distance of its Semidiameter, a white paper, in such manner that one may see the light of the Candle, which hath passed thorough the bottle, reunited in a little space upon the Paper; then they, who have their eyes near the Candle, shall see the bottle full of light, which will disappear, if you hold the paper either nearer to, or farther from the bottle. And if one hold a small lighted wax-candle in the place of the Paper, and you hold your eye in the place of the former candle, you shall see the bottle yet more enlightened than it was before: And one may easily judge, that the light which appeareth in the dogs eye, proceedeth from a like cause. The same experiment may be made in the the eyes of several other Animals, and particularly of Cats; in whose eyes this light appears bleuish, which shews, that it proceedeth from their Choroeides which hath much of this Colour; but this Colour, nor any other which may be in the Choroeides, brings no confusion to the Sense of Seeing, for the Senses receive no impression from their own Organs. The rest of this first Objection hath almost no other foundation, but an interpretation you give to some words of my writing different from my sense; for, when I said, that dark and opaque bodies receive a great deal of impression from the light; I did mean dark and opaque bodies both together; for it had been sufficient for me to name dark bodies, since all dark bodies are opaque; but my thought on this subject was, and still is, that transparent bodies, as the Ayre, the Water, and the Retina in a living Animal, receive little impression from light, and that Opaque bodies receive a great deal, but that dark bodies receive more than all other Opaque bodies, and the Air and the Water a little less than the Retina. Nor do I believe, that darkness is absolutely necessary for Vision, but only for a strong vision; nor that the picture of objects ought to be expressed on the Organ of Vision; for, it sufficeth, that the rays of each point of the Objects be reunited in a distinct and separated point, according as they answer one another; and you will easily agree, that as a Convex glass makes the image of the Sun to appear on white paper with a great deal of brightness and light, and on black paper very obscurely; although black paper (which soon takes fire) receives a great deal more impression than white: So the rays of illuminated objects do reunite on a whitish Choroeides, and express there a visible picture, and on a dark Choroeides, a very obscure one, and which cannot be seen; but then the impression is also much stronger in the black than in the white: And this is the cause, why Men and Birds see better and more distinctly than the greatest part of other Animals; for their Choroeides being black, and by consequence very sensible of light, they contract much their pupil-la or sight-hole of the Eye, which makes the Rays, that pass therefrom each point of the Object, to be all very near the Axis of the Chrystallin, and to reunite more exactly in a point, then in the Eyes of most other Animals, which have their Choreides white towards the Axis of Sight, and by consequence less sensible of light, who in recompence can very much dilate the Pupilla of their Eyes, when they stand in need of a great light; but also their Sight is not so distinct, because the Rayes, which fall on the Extremity of the Chrystalline, do intersect the Axis too near in their refraction. It is true, that to supply in some sort this defect, they have a little Chrystalline in the middle of the great one, and this little Chrystalline being of a more spiss consistence than the great one, its refraction is also more strong, and makes the rayes, which come from one point in the Axis and pass near the Center of the Chrystalline to retract more than if there had been but one Chrystalline. And by this means the greatest part of the Rayes, which fall upon the Extremity of the great Chrystalline, do intersect the Axis; which causes their sight to be less confused, although it be never so distinct as that of Men and Birds, which have but one Chrystalline. Fishes have also a double Chrystalline, for otherwise their sight would be more confused than that of other Animals who live in the Air, for their Chrystalline being spherical, the rayes cut the Axis more unequally than if it were lenticular and its convexities were of a greater sphere; and it ought necessarily to be spherical, because the refraction of the Rayes, which pass from the water into the Chrystalline, is very small, and would make its focus too far distant if the Chrystalline were lenticular. The difficulty of your Second objection proceeds also from an Ambiguity of words, and consists in knowing, what we are to say it is, that has a great continuity and communication with the Brain. My Hypothesis is, that the Nerves are all coated with the Pia Mater (which envelops all the Spinal Marrow) and have with it the same continuity of fibers; so that if these nerves be never so little moved, the impression is carried to the Brain by the Continuity of these fibers; and whether it be, that the texture is different in Nerves of different senses; or that they contain some spurious liquors which determine their sen- sensations, by some differences they have among themselves; it is certain, that the Nerves of Sight, in what manner soever they are moved, represent Colours and Lights; those of Hearing, Sounds, and those of the Touch, Paines, &c. Now the Choroides is an Expansion and dilatation of the Pia Mater, which envelops internally the Optique Nerve, and which comes from the tuberosity of the Spinal Marrow, by a continuity of fibers; whence it follows, that how little soever the Choroides be touched, the impression may be easily communicated to the brain: And to the end, that the same thing may be said of the Retina, there must be a little channel in the Optic nerve, through which the Retina in its proper Substance extends itself to this tuberosity by a continuity of Fibers; which is not seen, and you are constrained to say, that there are little filaments of nerves which come from the interior of the Optick, and expand themselves through the Retina, which have this continuity; but if there were these filaments, they should spread themselves through the Retina, as from a Center to a Circumference, and should lie closer together near the Optick nerve, than a good way farther in the Retina: which nevertheless we do not observe to be so. Besides, if you thrust a pin through the thickness of the Retina, you will often meet with filaments; but if you look on them through a Convex glass, you will discover, that they end in little vessels of Veins and Arteries, which are in the Retina; and infallibly, if there were any nerves, you might find them in the same manner, and they would stop the Pin, since they are as hard and firm as the little Arteries; and when you say, that one may distinguish these filaments in the water, because the rest of the Retina disappears, that is repugnant to experience, and to what you have said before, viz. that the Retina may be seen all white in the water, and without transparency; and you ought to shew these filaments, or else we shall take them for a thing invented at pleasure. You bring after that two Experiments; the first of which is, that if an aperture be made on the upper part of the Eye, we may discover the Picture of the Objects on the anterior surface of the Retina; but if this aperture be made in the white of the Eye, Eye; the aqueous humour will run to the Tunica Cornea, and make wrinkles, which will hinder the Picture from being distinct: Besides, he that looks in at this aperture, will hinder the Rayes of the Object from passing into the Eye, and he shall see nothing there but his own image. But if you mean, that the Tunica Cornea should be wholly taken off, there will not then be distance enough between the Chrystallin and the Retina, to make the Picture distinct. In conclusion, I do not believe this Experiment can be made, much less that it is to be discerned whether this Picture be formed on the anterious or posterious surface of the Retina, since the thickness of it is less than half a line, or the 24th part of an inch; and there is reason to believe, that you have trusted to the report of some other concerning this Experiment; or that you have believed the images which appear in the Eyes to be painted on the Retina, whereas they proceed from the reflexion made on the exterious of the Cornea. Your second Experiment is true and easy to be made, but according to you it were impossible; for, since you hold, that it is on the anterious part of the Retina, that the Picture is seen, and that you have elsewhere said, that one cannot well see through this Membrane, it follows, that you cannot see this Picture through the thickness of the Retina; but because I believe, there remains transparency enough in that part of the Retina which is not expos'd to the Air, I doubt not but the Picture may be seen on the posterious part of it, after it is become sufficiently opake, though in a living Animal this Picture passeth as far as the Choroides, as it hath been already proved: and if the Retina itself were taken away, and there remained only the vitreous humour, you might notwithstanding see the inverted Picture of the Windows toward the bottom of the Eye, if you held it at the farther side of the room; in the same manner as the Picture is to be seen in the Focus of the spherical Glass-bottle fill'd with water, though it seem to be on the exterious surface of the Glass; and by consequence this Experiment proves not the opacity of the Retina. In your third Objection you quote, what I said, a little otherwise than I spoke it; for I put in my Writing, that the Retina was about half a line in thickness, and not half a line precisely; which which shew'd, I had not measur'd it exactly; but if it were but a quarter of a line or less, it sufficeth that it had enough for the effect I did attribute to it: And for another, I did also mention in my Writing, which is, that the Rayes of the same luminous point, which do not unite precisely in the same points in the Axis, are rectified by the concavity of the Retina, those that are farthest off the Axis more than those which are the nearest, which is the reason they unite better in the same point on the Choerides; the which point I hold (with you) to be Physical, since the points objective are so too; but I hold it to be lesser than any that can be perceptible to the sight; for we distinguish the different parts of very small objects, as the extremities of the breadth of the little arteries of the Retina, which make not an eighth part of its thickness; and that which represents this small breadth, must be 25 or 30 times less in the Organ of our sight, that is to say, in the proportion of the distance of the Object, to the Centre of the sight, and of the distance of this Centre, to the Organ of vision; and by consequence the thickness of the Retina is not proper for this smallness. You see, then Sir, that hitherto your objections have been able but very lightly to shake my opinion, and that the transparency of the Retina is well enough established: Let us come now to the proof, I make of the Want of Vision, on the Basis of the Optick Nerve. It must first of all be agreed, that in this Experiment, almost all men do loose sight of an entire circle of white paper, whose diameter is about the 9th or 10th part of its distance from the Eye. Now the visual Triangle, whose Basis is the Diameter of the Circle, and whose top is the Center of vision, is proportioned to the Triangle, whose Basis is the Diameter of the picture of the Circle on the bottom of the eye, and the top, the same center of sight, the which center being distant 6 or 7 lines from the basis of the optick nerve, whose breadth is about 4 of a line, this basis also will be about the 9th or 10th part of its distance from the Center of sight, and by the Principles of Opticks, the Image of the circle of the white paper, falling on the basis of the optick nerve, will cover it precisely; and because the Paper then wholly disappears, it follows, that all the basis of the Optick nerve is insensible of light; whence I conclude, that the Choroeides is the Principal Organ of sight; and that the Retina is not, seeing it is placed in that part, and is there apparently disposed in like manner as to the rest of the bottom of the Eye. To elude the force of this Argument, you alledge other causes of this fault of sight; the two first are almost like one another. But it seems to me, that you suppose them without any ground: for, as I said before, there are no such filaments of nerves to be seen coming out of the basis of the optick nerve; besides, they are not proper for vision, since they would leave in the parts of the Retina too great intervals empty; and its necessary, that every point of the Object find a sensible point in the Organ of sight to unite his rays there; which is found in the Choroeides, which is an Expansion of the sensible part of the nerve into a continued membrane. Besides, the causes of the defect of vision cannot be found in these Hypotheses; for in the first, what reason can be given, that there should be no extremities of nerves opposite to the optick nerve. Since there would need only a simple direct continuation of some of its fibers unto the Anterior part of the Retina; and for the second (which is the particular opinion) I see not, how the void space in the middle of the inverted Tuft, can comprehend all that extent of the basis whence it proceeds, and why the filaments begin not to receive the impression of the luminous Rayes, but upon the Extremity of this basis, since the rays of a luminous point, which unite themselves in a point at the bottom of the Eye, come from divers points of the Chrystalline, and so, if one part of these rays were oblique, the other should be perpendicular, and if all the Rayes should fall perpendicular on the Organ of sight, there would be no vision, since there is never but one single Ray of those which unite in a point, that falls perpendicular. But I shall not inlarge myself to demonstrate these impossibilities, since I am perswaded, that these inverted tufts, and these filaments of which they are composed, are but things without a foundation, and which you would have much ado to shew us. The other Cause which you bring, is the trunk of the vessels which proceed from the basis of the nerve. Yet you cannot deny deny but they are very small, and that it is very hard to discern the little holes through which they pass, when the nerve is cut off above its insertion into the Eye; and because they often come out of the basis by two several little holes, the diameter of each of which does not take up the space of above the eighth part of the diameter of the basis; it follows, that if the rest of the nerve were sensible of light, we should not loose sight of a paper of two inches diameter at most, at ten foot distance: and sometimes in fixing one eye on a little piece of Paper, two other, very little ones, separated one from the other, would disappear; which is contrary to experience: for the default of vision is continued. Seeing than the reasons which you allegd of this default of sight are either insufficient, or want an assured foundation, it follows, that that which I propose must stand, at least, in respect of any thing you urge against it. And to confirme it yet more, I will adde here some reasons and observations, which are neither in my letter nor in my writing. The first observation which is very common, is that of the Pupilla dilates itself in the shade, and contracts itself in a great light, and it is very hard to find the cause of this involuntary motion, but by the Hypothesis, that the Choroeides is sensible of light, for then it is easy to conceive that it being hurt by too strong a vision, it may dilate or contract its fibers, which have one continuity with those of the forepart of the uvea, so that it can contract its aperture, and when it is not hurt, relax it again: whereas if the Retina be supposed to be the Organ of sight, it will be very difficult to explain how this contraction is made. The second is that of the glass bottle full of water, when a lighted candle is placed near its focus: for it is easy to prove that if one hold his hand between the Candle and the bottle, he will feel more heat than if he held it in the reciprocal Focus; that is to say, the place where the Rayes, which have passed thorough the bottle, make a great image of the flame of the Candle appear inverted upon a white surface opposed to it: for I draw this consequence, that the Image of a Candle, which is painted on the Choroeides of a dog, as I have prov'd to you, makes a much greater impression on the Retina of the dog, than on that of him that looks on it and sees it very bright; whence I conclude that if the Retina were the Organ of sight, this dog would not see the objects indifferently enlightened, which are about the candle, although at 3 or 4 foot distance, because they would receive a great deal more impression from this reflexion, and that a great sensation doth obliterate a lesser; which is contrary to experience, and it is not at all likely there should be such a defect in the sight of Animals. The third is, that the Eyes of Birds are so framed, that the Optique nerve after its insertion into the Eye is inflected, and extends itself on the Concavity of the sclerotis about the breadth of 2 or 3 lines more or less according to the bigness of the Eyes; and the length of this inflection is covered by the Choroeides, leaving but one little white streak in the middle, from whence the Retina takes its Original, which extends itself on the Choroeides through all the bottom of the Eye, but it is covered on the side of this white streak with a little black membrane as long as the inflexion of the nerve, and almost as broad; which proceeds from the pia mater, and is as it were an appendix of the Choroeides: and if you consider the situation of this membrane, you will find it is near the Axis of sight, and that the rays of the Objects, which the Birds look on with both Eyes, fall precisely upon it after their refraction. Since then in the place, where Vision ought to be strongest, the Retina is covered, and that no man doubts but Birds are more clear-sighted than other Animals; you ought to acknowledge, that the Retina is not the principal Organ of Vision, but that that preeminence belongs to the Choroeides. AN ACCOMPT Given by Doctor Edward Brown, concerning the Copper-mine at Herrn-ground in Hungary. Herrn-ground is a little town seated very high between two Hills, upon a part of land of the same name, an Hungarian mile distant from Newstol. In this town is the entrance into a large Copper-mine, very much digged. I went in through a Cuniculus, called Tach-stoln, and continued divers hours in the Mine, and visited many of the most remarkable