Philosophical Transactions/Volume 13/Number 147/A Continuation of a discourse about Vision, with an Examination of some late objections against it
HAving formerly given a [1]Specimen of my thoughts about Vision, I purpos'd to defer the publishing any thing more relating to it till a more entire Treatise (about the Particular uses of the parts of the Eye in Vision) had been finisht, which I intimated my designing at that time; but this will require the consideration of many more years, and indeed the Subject is so admirable, and may be of such use to convince the Scepticism of the age, that I hope it will not be time lost: and if I have sufficient opportunities, or be assisted with the practical Observations of others whom I dare confide in, I may perhaps add withall the Pathology of that Useful part, to make it the more acceptable to the World. In the mean time I have been prevailed with to make this inlargement of the forementioned discourse, in order to the fuller explaining my thoughts, and the clearing some difficulties which have been propounded against it.
In that small Essay I endeavour'd to shew. 1. That the fibres of the Optic Nerve, as rising from the two protuberances of the thalami Optici, were more concern'd in Vision then either the Cornea, Humours, or Retina (as they are consider'd by Writers in Optics); not only because sensation is perform'd chiefly in the Brain, and these other parts are but the transennæ to it; but also because in an Amaurôsis or gutta serena these parts are free from any indisposition (the Eye appearing, as naturally, without any fault), tho the sight is then wholly lost; and therefore those Fibres of the Optic Nerve must be principally affected, either by [2]being obstructed, or the roots of 'em comprest (about the thalami Optici, by some tumour, or too much pent in by a contraction and extenuation of the outward coats of the Nerves, or by any Confusion or Contortion of the said Fibres.
2. I shew there that the superior Fibre in each thalamus Opticus had the greatest tension, and the inferior the least; as may appear from the former arising from the top of the thalami optici and having the greatest flexure thus 
; and the latter arising from the lower part of the aforesaid thalami and having the least flexure thus 
, as may be judg'd from a view of those parts in the Brain; so that the Correspondence of the former or latter in site and tension caus'd that correspondence or Union in Vision.
3. It may be further noted that the intermediate or lateral Fibres in the same Eye, tho diametrically opposite to one another, are said to differ in tension (by reason of a more considerable flexure of the external then the internal;) whence two Stars or other bodies seen by the collateral Fibres of the same Eye (whilst t' other may be shut) appear distinct and not as one, because they are viewed by discord Fibres of that same Eye; which likewise is so kept in its Orbit by the investing parts, that it can't well be otherwise.
4. I observ'd that the Optic Nerves arose[3] separately from those two Moleculæ of the brain, and besides have a peculiar advantage in rising from these hillocks in this manner; whereas the other Nerves arise from the basis of the brain in a flatter manner, and closer together, so that the extream difference of their rise is very remarkable, and intended surely by Nature for some extraordinary end. Thus that of the optic Nerves in order to their consent requir'd a corresponding tension in their Fibres, or else Vision had been always double, since those Nerves arise separately from two distinct eminences of the brain: but the other Nerves arising closer together (whereby there may be a communication between their Fibres) or belonging to senses that require not so minute discrimination of objects as Vision does, there needed not that exactness in the placing or tension of the Fibres.
5. In the position of the Fibres of the Optic Nerves I fhew that they keep their distinct order, and consequently that they are not mixt or blended together at the place of their connexion (as was frivolously suppos'd by Authors before to solve the Union of Vision). And this I observ'd not only from what is noted in Man by Vesalius, Riolan, and others in their particular remarks in this Case, but also by what Nature it self shews in the Chamæleon and several of the feebler sort of Fishes, where they are scarce join'd; and this she might intend in these small or helpless creatures (who turn thereby their Eyes to several objects and different coasts at once.) to avoid those on all sides that infest them, or to catch the more readily their fleeting food; whereas others that have 'em closer join'd view particular objects the better by looking more steadily or intently toward one part, and are otherwise provided for in their food or their safety.
6. I observ'd that in the insertion of these Fibres into the Eye (where the Medullary part of them forms the Retina) they still kept their distinct series, and that they are much kept in, not only by being fastne'd (or terminating) on the processus ciliares, but also by little transverse Fibres (that are not described in the Figure I have given) which serve to connect those that run [4]long-waies, there described's and make the whole Coat appear in a glass cf clear water like Lawn or Tiffany as I have shewn. None that I know ever did it before me, [5]and those that have mention'd the same experiment since (without taking notice hereof) have mistook my intent in it. For the putting the Retina in water is not to wash off the mucous Substance, which is its proper Substance, but 'tis to expand the Fibres by the playing it up and down in warm water, and to magnifie the Image of it by a double refraction of the lucid raies, which pass through that and the Glass that contains it.
That there is a little white slimy matter comes off upon washing the Retina is true, and this serves to fill up the interstices of the Fibres and thicken the Coat, whereby the Raies terminate the better, and pass not through to the Choroeides, (which takes off in some measure Monsr. Mariotte's objection of which more anon) and this may be part of the succus Nutritius of the Nerve; tho however the Coat may be as well said sistere species (as they call it), or to terminate the Raies, as the Oil'd Paper does the turning Images in the Lantern, notwithstanding it be in some measure diaphanous. Besides toward the bottom of the Eye the Fibres of this Coat converge very much or come closer together, and 'tis here that is the most lively representation or exquisitest sense of the object, for wch reason partly, as also partly from it's figure, I formerly took the liberty of calling it a Pupilla inverted. This lies in Men diametrically opposite[6] to the Pupill, as the Optic Nerve is plac'd in the forementioned figure: but in Brutes more obliquely by reason of the insertion of the Optic Nerve more toward the inner Canthus; so that sometimes (as we may see in Horses upon starting} they are forc't to turn their Eye accordingly, to distinguish clearly objects that surprize them.
7. But next of all I would have it observ'd, that whereas I say the intermediate Fibres gradually differ in tension as they are nigher or further from the top of the Thalami Optici, it may be easily suppos'd that they do it by so Minute Gradations, that the difference of those that are nigher to the Top, from the Superior of all, is very little (and therefore cannot make so considerable a difference in the view of the parts of an-Object), but from those that are further off great enough, and the difference of the highest fibres from the iowest, greatest of all. Besides I would have it observed that 'tis the different tension of the Thalami Optici, and not so much a varying Expansion of them in the Eye, that makes the difference. For as the Eye discerns an Object more by the inward than outward cone of Vision: so the Soul may be well supposed to judg of or discriminate things abroad, not so much by the outward part of the fibres inserted in the Organ, as by the inward that terminate about the common Sensory in the brain and more immediately affect her.
8. Whereas I mention sometimes the parallelisme of the Correspondent fibres, I mean it not in a strict Mathematical Sense (as I partly hinted at the latter end of that Essay,) but only their being as it were in æquilibrio or due poise in respect of their situation; and therefore if those fibres had been straight (and not of a Curv'd figure, as they are) I should have rather chose to have exprest my mind by the phrase of Mathematicians, of their being in eodem plano. But my sense being understood there need not be any exception to the word, since it was not so easy to express my meaning by a better; and therefore I shall pass by this, and proceed to more real objections that have been sent me by Mr. Newton our worthy Prof. of Mathematicks at Cambridge (and other friends,) relating to the Opinion it self.
The i. Objection was made in the R. S. when it was read there, which (as I was told) was this; viz. That it seem'd difficult to conceive how those soft Medullary Fibres of the Nerve could have such a tension. But this is not harder to conceive than in that of a Spiders-Web, whose Mucous substance and Expansion very well answers to that of the Retina (whilst in its due position or Expansion in the Eye;) and as the least breath of Wind moves the one, so the least gale of the Etherial or lucid matter causes a vibration in the other.
Further it was objected, That it was hard to conceive how soft a body as the thalamus Opticus (being only a protuberance of the Medullary part of the Brain) could make such a difference in the Stress or tension of the fibres: But 'tis apparent that upon drawing the Nerve from it forward (according as 'tis situated and runs toward the Eye) the Superior fibres are more upon the Stress than the lateral, and Nature in these cases is finer in her operations (or to speak more properly the great Author of Nature is) then we are in our Conceptions of them. It seems so especially in the formation of this Organ, where the Apparatus of its parts in order to vision is so curiously contrive'd by the great Artist and all is done as it were in so fine Miniature and with so soft Touches of his inimitable hand, that it exceeds as much the other parts of the human body, as that does eminently transcend the remaining frame of the visible World.
2. It has been objected by others, That if the Superior fibres were more tense then the inferior we should see better by raies falling on the top than the bottom of the Eye, or see an object better p1ac't below our Eye (when the raies passing in a straight line from it must terminate in the top fibres) then above it, when vice versâ they must terminate in the lower fibres. To this I answer, that it does accordingly fall out so, and this is a more positive and direct proof of my Opinion; for I appeal to any man's experience whether the Characters in a Book appear not better to him, or he reads not better in it held about half a yard under his Eye than so much above it; or whether he does not more readily discern or find out objects beneath than above him with the same light: and this may be further illustrated by my [7]Ld, Bacons experiment of a mans appearing better on the ground to him that is plac't on a high steeple then vice versâ.
3. It is urg'd That according to my Scheme of the situation of the correspondent fibres, the Raies of an object plac't laterally (suppose toward the left Eye) could not fall upon the fellow Fibres in the right Eye; for if it were plac't so slantingly toward the left Eye the raies could not fall upon the internal lateral Fibres of both Eyes in that position; but upon the internal of one, suppose the left Eye, and the external lateral of the right; which would cause a double perception. This Objection I foresaw when I hinted (in p. 176. of the aforesaid Collections) that Whether the Nerves decussated or not, it would be no prejudice to my Opinion, nay perhaps might more fully confirm my opinion where they do. In that passage I had respect also I confess to the inversion of the Image in the Eye being rectified in the Brain; tho that equally presses any other Hypothesis, and the explication of the thing may be well enough understood by a blind man's judging of the position of an object above his head by touching it with one end of his stick, tho the other end terminates under that Object or in his hand: And so in our view of an object the true situation of the respective parts is not distinguish't so much by the means of that end of the ray that terminates in the Eye as of t' other end that touches the Object, from whence the vibration or protrusion comes.
But to leave this which does not so particularly concern me, I come to the objection it self as it relates to my Opinion; and tho it seems at first view the most difficult of all to be answer'd, yet it may be determin'd by the Experiment it self, better than by the Scheme (in the Philos. Collections), where the Eyes are not drawn in that position that is here requir'd. Now let there be plac't an object near the left Eye of any person (but not so near that Eye as that the Nose might hinder the rays from falling on the right, because it is to be seen with both), and whilst that person looks on it let a By-stander observe the position of both Eyes, and he shall see that the pupil of the right Eye is turn'd in a very oblique manner to the object, whereas the pupil of the left is scarce so at all, whereby there will be three parts to one more in the distance of the pupil of the right Eye from the external Canthus (as may be judg'd by the proportion of the White that appears) then there will be in the other; so that the position of the right Eye in respect of the left is as in Fig. 5.
b. The left Eye.
c. The right.
d. The Pupils.
e. e. Two internal-lateral Fibres.
f. f. Two external-lateral.
g. g. The Optic Nerves.
Hereby it appears that if the Object be so plac't that it is seen with both Eyes, the right Eye accommodates it self to the position of the left, that the rays strike correspondent Fibres, and the percussion or Vibration being toward the bottom or Papilla of the Eye (or near its Axis) where I before observ'd Vision to be chiefly performed, a small turning of one Eye to another will make that accommodation.
Moreover as this accommodation is made in an oblique position of the object, so is it more readily done in a direct position of the same; and this we may perceive in an Object's retiring in a straight line from the Eyes, whereby the Pupils gradually devaricate; as on the contrary, they converge when the Object is seen very near them, and that so forcibly that 'tis a pain to hold them long in that posture. Now by this various incidence of the rays sometimes on the internal and sometimes external Fibres (according as the Object approaches or recedes from us) its varying position in respect of distance from us, is perceived, tho it recedes from us in a straight line, and at the same time be equi-distant from the Horizon with our Eyes. In Brutes also we see their Eyes accommodate themselves in their viewing a single object: But however the rays seem to fall here more readily upon the external Fibres, because of the oblique insertion of the optic Nerve, and therefore they do not so indifferently turn their Eyes to discriminate the Motion of objects as we do, nor is their Sphere of Vision so large.
4. But to proceed to other objections; The case of Cross'd-ey'd persons by birth (that are so from a small contortion of one of the Motory Muscles of the Eye) I have confider'd at the end of the foremention'd [8]Essay, and shewn withal why a Morbid strabismus, or more violent contortion of those Muscles after great convulsions of the Nerves, causes always a double Vision (to which the instances out of Dr. Willis, &c. In p. 176. of those Collections do refer;) which problem was so hard to consider of by Plempius, and others before; and indeed can scarce be solv'd by any other Hyothesis.
5. It has been urg'd That the tension of all the Fibres of the Optic Nerves might be uniform, notwithstanding the greater flexure of the Superior; because these latter might be longer, and consequently might not have a greater stress upon the thalami Optici then the lateral: As, if the arm of a tree grows bent, the Fibres on the protuberant part seem not more stretcht then on the concave side, but to take only a longer compass. To which I answer that sense evinces the contrary in our case (as I shew before), and if any one draws out the Optic Nerve straight forward from the thalamus Opticus, or as it lies in its Natural position, he will plainly see that the top Fibres press more on the subjacent medullary Protuberances then the lateral or, make a deeper impression. Besides to answer one similitude with another, we may observe that the Fibres of those Muscles that extend the leg, and bear-upon the bow of the knee, seem more strecht and vigorous in their action then the contractors that run in the hollow under it; and this was admirably contriv'd by the Wisdom of our great Author (and may be unanswerably objected to the followers of Epicurus, who say the parts of the human body were made without any design:) for since those Extending Muscles of our leggs are much pent up and hindred in their action by the posture of the Child in the Womb (which lies with its knees up to its mouth,) that defect is excellently compensated by the natural tension and position of the said Fibres, or else Man could never go upright.
6. It has been urg'd That the action of Vision was uniform, and therefore requir'd an uniform tension of all the Fibres. To which I answer, that tho in the view of the intire object, or its place lay both eyes, it ought to be so, and that therefore it was done by correspondent Fibres (as I have formerly explain'd); yet in a stricter view of the parts of the same Object by one Eye, there is a discrimination. For to instance in a body of the most simple figure and colour, (as suppose a Globe all red, or of Fire) that should be seen by one Eye only, 'tis certain the Eye distinguishes the different parts in their extra positions, or distinct situations in respect of one another, tho they be all of a piece as it were otherwise: and unless it were so, I see no reason why that red Globe should not appear only one red speck, Or a Globe of fire as a lucid point: Now this distinguishing of the parts is easily conceivable to be done by the discord Fibres of the same Eye.
7. Lately it has been urg'd That the Fibres of the Choroeides seem more adapted to Vision then those of the Retina, because these last did not sistere species (to use their phrase) as transmitting the colours of the former; and besides some Blood-vessels running amongst 'em would interrupt the image; and lastly sensation could be better continued to the Tense Fibres of the Pia mater by the one, then to the brain by the softer of the other.
Although this last Objection does not directly strike at my Notion of Vision (because a correspondence of Fibres may be understood as well in one as t' other); yet it may not be amiss to consider it particularly; because I have formerly asserted and do still, that Vision can be no way better performed then by the Fibres of the Retina, however other senses may require in their action a greater stiffness in the Membranes that are subservient to them, which some of late will have to be the only instruments of sense. First then it is certain that (as I said before) the Retina is no more transparent if so much as the oiled paper in the lantern, which yet serves well enough to intercept the turning images of it. Secondly, That being of a whitish colour, and resembling thereby the white Paper in the dark house, it is fitter to receive the images of colour'd objects then the dark shade of the Choroeides. Thirdly, It being the more inward or medullary expansion of the Optic Nerve it can more immediatly transmit any motions to the Meditullium of the brain (or the common sensory) then the other part, which by its continuation to the pia mater does not reach it; and this I urg'd formerly, which has not yet been answer'd by any of Monsieur Mariotte's followers. Fourthly, The blood-Vessels running upon it is as well an objection against the Choroeides (if the latter be not chiefly a Plexus of the same as has been lately well argued), because this Coat lies under the Retina, and consequently under them too and therefore hereby is only prov'd that in some positions of our body, or in some Stations, we do not so well view an Object as in others, and this is very true. Fifthly, As to the Tensness of the Fibres, I before observ'd that the Retina has as much as those of a Spider's Web, and this is sufficient, nay more suitable to the finer stroaks of the etherial or lucid matter and the nice actings of this sense, which is not rerequired in any other: and therefore the same objection may lye against the Constitution of the brain it self, which consists of soft medullary Fibres that are however fit enough to receive or propagate any motion, and whilst they are fill'd with Animal spirits may be allow'd to have the like Tensness, or resistance that a lock of Wool has, or a Spider's Web. And if I may be admitted to carry on the similitudes; As that little Animal in the Centre of its soft circumtended Fibres is sensible of the least gale of Wind, or is alarum'd by the least noise or touches of its prey or of an enemy from any Quarter, by the delicate expansion of its Fibres: So may the Soul much more (in the common sensory) being surrounded by Fibrillæ of expanded Nerves and of a finer make, apprehend from what Quarter the several motions come from abroad, and more minutely perceive the difference of 'em in respect of the diverse Organs of sense and the different fineness or tension of those Nerves that belong to the same.
SOme faults being committed by the Press in the Previous Discourse to this, in the Philos. Collections, Numb. 6. may be thus corrected. Pag. 170, lin. 29, for even and sight, read even and in situ. p. 173. l. 28, for sight. r. site. In the Margents of pag. 170. 173. 176. to Fig. 1, add the Characters i, i.
- ↑ In Mr. Hook's Philos. Collections No. 6.
- ↑ See Bonet. Sepulchret, tom. i, lib. i. Sect. 18. Observ. i., 3. & 5.
- ↑ See Fig. i. in the fore-mentioned Collect. l. l.
- ↑ See the i.Figure in the former Collections, where the Letters a, b, c, d, & e. in the Eye shew these direct Fibres of the Retina.
- ↑ In Ophth. p, 30, published A. D. 1676.
- ↑ Fig. i. In those Philos. Collections.
- ↑ Experim (illegible text) Cent. 3. Nat Hist.
- ↑ In p.177. of the Philos. Collections No. 6.