No. VI.

(Referred to in page 226.)

NEWTON'S Fifteenth QUERY.1

ARE not the species of objects seen with both eyes united where the optic nerves meet before they come into the brain, the fibres on the right side of both nerves uniting there, and, after union, going thence into the brain in the nerve which is on the right side of the heart; and the fibres on the left side of both nerves uniting in the same place, and, after union, going into the brain in the nerve which is on the left side of the head, and these two nerves meeting in the brain, in such a manner that their fibres make but one entire species or picture, half of which on the right side of the sensorium, comes from the right side of both eyes through the right side of both optic nerves, to the place where the nerves meet, and from thence on the right side of the head into the brain; and the other half on the left side of the sensorium comes in like manner from the left side of both eyes. For the optic nerves of such animals as look the same way with both eyes, (as of men, dogs, sheep, oxen, &c.,) meet before they come into the brain, but the optic nerves of such animals as do not look the same way with both eyes, (as of fishes and of the chameleon,) do not meet, if I am rightly informed.

1 Optics, 3d edit. 1721, pp. 320, 321.

No. VII.

(Referred to in page 226.)

ALTHOUGH We have extracted a part of this document in the text, for the sake of illustration, we shall give now the whole of it as published by Mr. Harris.

DESCRIPTION OF THE OPTIC NERVES AND THEIR JUNCTURE IN THE BRAIN, BY SIR ISAAC NEWTON.

The tunic retina grows not from the sides of the optic nerve, (as the other two which rise one from the dura, the other from the pia mater :) but it grows from the middle of the nerve, sticking to it all over the extremity of its marrow. Which marrow, if the nerve be any where cut cross-wise betwixt the eye and the union of the nerves, appears full of small spots or pimples, which are a little prominent, especially if the nerve be pressed, or warmed at a candle; and these shoot into the very eye, and may be seen within side, where the retina grows to the nerve; and they also continue to the very juncture E F G H. But at the juncture they end on a sudden into a more tender white pap, like the anterior part of the brain: and so the nerve continues after the juncture into the brain filled with a white tender pap, in which can be seen no distinction of parts as betwixt the said juncture and the eye.

"Now I conceive that every point in the retina of one eye, hath its correspondent point in the other; from which two very slender pipes filled with a most limpid liquor do, without either interruption or any other unevenness or irregularity in their process, go along the optic nerves to the juncture E F G H, where they meet either betwixt G F or FH, and there unite into one pipe as big as both of them; and so continue in one, passing either betwixt IL or MK, into the brain, where they are terminated perhaps at the next meeting of the nerves betwixt the cerebrum and cerebellum, in the same order that their ex

tremities were situated in the retina's. And so there are a vast multitude of these slender pipes which flow from the brain, the one half through the right side nerve IL, till they come at the juncture G F, where they are each divided into two branches, the one passing by G and T to the right side of the right eye AB,

ß.

the other half shooting through the space E F, and so passing by x to the right side of the left eye a B. And in like manner the other half shooting through the left side nerve M K, divide themselves at FH, and their branches passing by EV to the right eye, and by н Y to the left, compose that half of the retina in both eyes, which is towards the left side CD and y d.

"Hence it appears, 1. Why the two images of both eyes make but one image a b c d in the brain.

"2. Why, when one eye is distorted, objects appear double; for if the image of any object be made upon a in the one eye, and ẞ in the other, that object shall have two images in the brain at a and b. Therefore the pictures of any object ought to be made upon the corresponding points of the two retinas; if upon a in the right eye, then upon a in the left; if upon B, then also upon B. And so shall the motions concur after they

VOL. I.

2 E

have passed the juncture GH, and make one image at a or b more vivid than one eye alone could do.

3. Why, though one thing may appear in two places by distorting the eyes, yet two things cannot appear in one place. If the picture of one thing fall upon A, and of another upon a, they may both proceed to p, but no farther; they cannot both. be carried on the same pipes pa into the brain; that which is strongest or most helped by phantasy will there prevail, and blot out the other.

"4. Why, if one of the branches of the nerve beyond the juncture, as at G F or FH, should be cut, that half of both eyes towards the wounded nerve would be blind, the other half remaining perfect.

"5. Why the juncture is almost as broad again betwixt G and H, as between E and F; because all the tubuli of both eyes pass between G and H, and but half of them betwixt E and F.

6. Why the nerve GILF buts not directly upon the nerve X EH Y, but deviates a little towards T v; because its tubuli are to pass only into that side of the nerve E H X Y towards EX. The like of FM K H.

66

7. Why the marrow of the nerve T VEG grows soft on a sudden, when it comes at the juncture E F, and more suddenly on that side towards G than towards E. And the like of the nerve EXYH: For it being necessary that the nerve TVEG should be stretched and bended several ways by the motion of the eye; therefore the tubuli are involved or wrought up within the substances of several tough skins, which, being folded up together, compose the marrow of the nerve pretty solid and flexible, lest the tubuli should be prejudiced by the several motions of the nerve. And those small pimples or prominencies which appear in the nerve cut crosswise, I conceive to be made by the foldings of those crasser skins. But the nerve at the juncture EG FH, being well guarded from all violence and motion by the bones into which it is closely adapted ; 'tis not necessary the said membraneous substance should be continued any farther than EG; therefore the tubuli there on a sudden unsheath themselves, that those on the inner side of the nerves towards v E and X E may severally cross 'twixt E F, and be united

with their correspondents on the other sides Y H and T G. Now, because the inner tubuli must first cross, before they can concur with the outmost tubuli of the opposite nerve; hence it is, that the nerves grow soft sooner on the inner side at E, than on the outer side at G and H.

"8. Why the two nerves meet a second time in the brain: because the two half images carried along IL and м K may be united in one compleat image, in the sensory. Note, that the nerves at their meeting, are round about disjoined from the rest of the brain; nor are they so thick there, as a little before their meeting. But by their external figure, they seem as if the capillamenta concentered like the radii of a hemisphere to a point in the lower part of the juncture. And 'tis probable, that the visive faculty is there for else why do the nerves swell there to so great a bulk, as it were preparing for their last office? Why do they run directly cross from either side the brain to meet there, if the design was to have the motions conveyed by the shortest cut from the eye to the sensorium, before they grew too weak. If they were to proceed farther, they might have gone a shorter cut, and in a less channel. There is indeed a marrow shoots from under them towards the cerebellum, to which they are united; but the greatest part of their substance, if not all of it, lies above this marrow, and also shoots cross beyond it to the center of the brain where they meet. Lastly, the substance here is most pure, the situation in the midst of the brain, constituting the upper part of that small passage 'twixt all the ventricles, where all superfluous humours have the greatest advantage to slide away, that they may not incumber that precious organ.

Light seldom strikes upon the parts of gross bodies, (as may be seen in its passing through them ;) its reflection and refraction is made by the diversity of æthers; and therefore its effect upon the retina can only be to make this vibrate which motion then must be either carried in the optic nerves to the sensorium, or produce other motions that are carried thither. Not the latter, for water is too gross for such subtile impressions; and as for animal spirits, tho' I tied a piece of the optic nerve at one end, and warmed it in the middle, to see if any airy substance by that means would disclose itself in bubbles at the