Micrographia/Chapter 39
Observ. XXXIX.Of the Eyes and Head of a Grey drone-Fly, and of several other creatures.
I took a large grey Drone-Fly, that had a large head, but a small and slender body in proportion to it, and cutting off its head, I fix'd it with the forepart or face upwards upon my Object Plate (this I made choice of rather then the head of a great blue Fly, because my enquiry being now about the eyes, I found this Fly to have, first the biggest clusters of eyes in proportion to his head, of any small kind of Fly that I have yet seen, it being somewhat inclining towards the make of the large Dragon-Flies. Next, because there is a greater variety in the knobs or balls of each cluster, then is of any small Fly.) Then examining it according to my usual manner, by varying the degrees of light, and altering its position to each kinde of light, I drew that representation of it which is delineated in the 24. Scheme, and found these things to be as plain and evident, as notable and pleasant.
First, that the greatest part of the face, nay, of the head, was nothing else but two large and protuberant bunches, or prominent parts, A B C D E A, the surface of each of which was all cover'd over, or shap'd into a multitude of small Hemispheres, plac'd in a triagonal order, that being the closest and most compacted, and in that order, rang'd over the whole surface of the eye in very lovely rows, between each of which, as is necessary, were left long and regular trenches, the bottoms of every of which, were perfectly intire and not at all perforated or drill'd through, which I most certainly was assured of, by the regularly reflected Image of certain Objects which I mov'd to and fro between the head and the light. And by examining the Cornea or outward skin, after I had stript it off from the several substances that lay within it, and by looking both upon the inside and against the light.
Next, that of those multitudes of Hemispheres, there were observable two degrees of bigness, the half of them that were lowermost, and look'd toward the ground or their own leggs, namely, C D E, C D E being a pretty deal smaller then the other, namely, A B C E, A B C E, that look'd upward, and side-ways, or foreright, and backward, which variety I have not found in any other small Fly.
Thirdly, that every one of these Hemispheres, as they seem'd to be pretty neer the true shape of a Hemisphere, so was the surface exceeding smooth and regular, reflecting as exact, regular, and perfect an Image of any Object from the surface of them, as a small Ball of Quick-silver of that bigness would do, but nothing neer so vivid, the reflection from these being very languid, much like the reflection from the outside of Water, Glass, Crystal, &c. In so much that in each of these Hemispheres, I have been able to discover a Land-scape of those things which lay before my window, one thing of which was a large Tree, whose trunk and top I could plainly discover, as I could also the parts of my window, and my hand and fingers, if I held it between the Window and the Object; a small draught of nineteen of which, as they appear'd in the bigger Magnifying-glass to reflect the Image of the two windows of my Chamber, are delineated in the third Figure of the 23. Scheme.
Fourthly, that these rows were so dispos'd, that there was no quarter visible from his head that there was not some of these Hemispheres directed against, so that a Fly may be truly said to have an eye every way, and to be really circumspect. And it was further observable, that that way where the trunk of his body did hinder his prospect backward, these protuberances were elevated, as it were, above the plain of his shoulders and back, so that he was able to see backwards also over his back.
Fifthly, in living Flies, I have observ'd, that when any small mote or dust, which flies up and down the air, chances to light upon any part of these knobs, as it is sure to stick firmly to it and not fall, though through the Microscope it appears like a large stone or stick (which one would admire, especially since it is no ways probable that there is any wet or glutinous matter upon these Hemispheres, but I hope I shall render the reason in another place) so the Fly presently makes use of his two fore-feet in stead of eye-lids, with which, as with two Brooms or Brushes, they being all bestuck with Brisles, he often sweeps or brushes off what ever hinders the prospect of any of his Hemispheres, and then, to free his leggs from that dirt, he rubs them one against another, the pointed Brisles or Tenters of which looking both one way, the rubbing of them to and fro one against another, does cleanse them in the same manner as I have observ'd those that Card Wool, to cleanse their Cards, by placing their Cards, so as the teeth of both look the same way, and then rubbing them one against another. In the very same manner do they brush and cleanse their bodies and wings, as I shall by and by shew; other creatures have other contrivances for the cleansing and cleering their eyes.
Sixthly that the number of the Pearls or Hemispheres in the clusters of this Fly, was neer 14000. which I judged by numbering certain rows of them several ways, and casting up the whole content, accounting each cluster to contain about seven thousand Pearls, three thousand of which were of a size, and consequently the rows not so thick, and the foure thousand I accounted to be the number of the smaller Pearls next the feet and proboscis. Other Animals I observ'd to have yet a greater number, as the Dragon-Fly or Adderbolt: And others to have a much less company, as an Ant, &c. and several other small Flies and Insects.
Seventhly, that the order of these eies or Hemispheres was altogether curious and admirable, they being plac'd in all kind of Flies, and aerial animals, in a most curious and regular ordination of triangular rows, in which order they are rang'd the neerest together that possibly they can, and consequently leave the least pits or trenches between them. But in Shrimps, Crawfishes, Lobsters, and such kinds of Crustaceous water Animals, I have yet observ'd them rang'd in a quadrangular order, the rows cutting each other at right angles, which as it admits of a less number of Pearls in equal surfaces; so have those creatures a recompence made them, by having their eyes a little movable in their heads, which the other altogether want. So infinitely wise and provident do we find all the Dispensations in Nature, that certainly Epicurus, and his followers, must very little have consider'd them, who ascrib'd those things to the production of chance, that wil, to a more attentive considerer, appear the products of the highest Wisdom and Providence.
Upon the Anatomy or Dissection of the Head, I observ'd these particulars:
First, that this outward skin, like the Cornea of the eyes of the greater Animals, was both flexible and transparent, and seem'd, through the Microscope perfectly to resemble the very substance of the Cornea of a man's eye; for having cut out the cluster, and remov'd the dark and mucous stuff that is subjacent to it, I could see it transparent like a thin piece of skin, having as many cavities in the inside of it, and rang'd in the same order as it had protuberances on the outside, and this propriety, I found the same in all the Animals that had it, whether Flies or Shell-Fish.
Secondly, I found that all Animals that I have observ'd with those kind of eyes; have within this Cornea, a certain cleer liquor or juice, though in a very little quantity, and,
I observ'd thirdly, that within that cleer liquor, they had a kind of dark mucous lining, which was all spread round within the cavity of the clutter, and seem'd very neer adjoining to it, the colour of which, in some Flies, was grey; in others, black, in others red; in others, of a mix'd colour; in others, spotted; and that the whole clusters, when look'd on whilst the Animal was living, or but newly kill'd, appear'd of the same colour that this coat (as I may so call it) appear'd of, when that outward skin, or Cornea, was remov'd.
Fourthly, that the rest of the capacity of the clusters was in some, as in Dragon Flies, &c. hollow, or empty; in others fill'd with some kind of substance; in blue Flies, with a reddish musculous substance, with fibres tending from the center or bottom outwards; and divers other, with various and differing kinds of substances.
That this curious contrivance is the organ of sight to all those various Crustaceous Animals, which are furnish'd with it, I think we need not doubt, if we consider but the several congruities it has with the eyes of greater creatures.
As first, that it is furnish'd with a Cornea, with a transparent humour, and with a uvea or retina, that the Figure of each of the small Hemispheres are very Spherical, exactly polish'd, and most vivid, lively and plump, when the Animal is living, as in greater Animals, and in like manner dull, flaccid, and irregular, or shrunk, when the Animal is dead.
Next, that those creatures that are furnish'd with it, have no other organs that have any resemblance to the known eyes of other creatures.
Thirdly, that those which they call the eyes of Crabs, Lobsters, Shrimps, and the like, and are really so, are Hemispher'd, almost in the same manner as these of Flies are. And that they really are so, I have very often try'd, by cutting off these little movable knobs, and putting the creature again into the water, that it would swim to and fro, and move up and down as well as before, but would often hit it self against the rocks or stones; and though I put my hand just before its head, it would not at all start or fly back till I touch'd it, whereas whil'st those were remaining, it would start back, and avoid my hand or a stick at a good distance before it touch'd it. And if in crustaceous Sea-animals, then it seems very probable also, that these knobs are the eyes in crustaceous Insects, which are also of the same kind, onely in a higher and more active Element; this the conformity or congruity of many other parts common to either of them, will strongly argue, their crustaceous armour, their number of leggs, which are six, beside the two great claws, which answer to the wings in Insects; and in all kind of Spiders, as also in many other Insects that want wings, we shall find the compleat number of them, and not onely the number, but the very shape, figure, joints, and claws of Lobsters and Crabs, as is evident in Scorpions and Spiders, as is visible in the second Figure of the 31. Scheme, and in the little Mite-worm, which I call a Land-crab, describ'd in the second Figure of the 33. Scheme, but in their manner of generation being oviparous, &c. And it were very worthy observation, whether there be not some kinds of transformation and metamorphosis in the several states of crustaceous water-animals, as there is in several sorts of Insects; for if such could be met with, the progress of the variations would be much more conspicuous in those larger Animals, then they can be in any kind of Insects our colder Climate affords.
These being their eyes, it affords us a very pretty Speculation to contemplate their manner of vision, which, as it is very differing from that of biocular Animals, so is it not less admirable.
That each of these Pearls or Hemispheres is a perfect eye, I think we need not doubt, if we consider onely the outside or figure of any one of them, for they being each of them cover'd with a transparent protuberant Cornea, and containing a liquor within them, resembling the watry or glassie humours of the eye, must necessarily refract all the parallel Rays that fall on them out of the air, into a point not farr distant within them, where (in all probability) the Retina of the eye is placed, and that opacous, dark, and mucous inward coat that (I formerly shew'd) I found to subtend the concave part of the cluster is very likely to be that tunicle or coat, it appearing through the Microscope to be plac'd a little more than a Diameter of those Pearls below or within the tunica cornea. And if so, then is there in all probability, a little Picture or Image of the objects without, painted or made at the bottom of the Retina against every one of those Pearls, so that there are as many impressions on the Retina or opacous skin, as there are Pearls or Hemispheres on the cluster. But because it is impossible for any protuberant surface whatsoever, whether sphærial or other, so to refract the Rays that come from farr remote lateral points of any Object as to collect them again, and unite them each in a distinct point, and that onely those Rays which come from some point that lies in the Axis of the Figure produc'd, are so accurately refracted to one and the same point again, and that the lateral Rays, the further they are remov'd, the more imperfect is their refracted confluence; It follows therefore, that onely the Picture of those parts of the external objects that lie in, or neer, the Axis of each Hemisphere, are discernably painted or made on the Retina of each Hemisphere, and that therefore each of them can distinctly sensate or see onely those parts which are very neer perpendicularly oppos'd to it, or lie in or neer its optick Axis. Now, though there may be by each of these eye-pearls, a representation to the Animal of a whole Hemisphere in the same manner as in a man's eye there is a picture or sensation in the Retina of all the objects lying almost in an Hemisphere; yet, as in a man's eye also, there are but some very few points which liyng in, or neer, the optick Axis are distinctly discern'd: So there may be multitudes of Pictures made of an Object in the several Pearls, and yet but one, or some very few that are distinct; The representation of any object that is made in any other Pearl, but that which is directly, or very neer directly, oppos'd, being altogether confus'd and unable to produce a distinct vision.
So that we see, that though it has pleas'd the All-wise Creator, to indue this creature with such multitudes of eyes, yet has he not indued it with the faculty of seeing more then another creature; for whereas this cannot move his head, at least can move it very little, without moving his whole body, biocular creatures can in an instant (or the twinkling of an eye, which, being very quick, is vulgarly used in the same signification) move their eyes so as to direct the optick Axis to any point; nor is it probable, that they are able to see attentively at one time more then one Physical point; for though there be a distinct Image made in every eye, yet 'tis very likely, that the observing faculty is only imploy'd about some one object for which they have most concern.
Now, as we accurately distinguish the site or position of an Object by the motion of the Muscles of the eye requisite to put the optick Line in a direct position, and confusedly by the position of the imperfect Picture of the object at the bottom of the eye; so are these crustaceous creatures able to judge confusedly of the position of objects by the Picture or impression made at the bottom of the opposite Pearl, and distinctly by the removal of the attentive or observing faculty, from one Pearl to another, but what this faculty is, as it requires another place, so a much deeper speculation. Now, because it were impossible, even with this multitude of eye-balls, to see any object distinct (for as I hinted before, onely those parts that lay in, or very neer, the optick Lines could be so) the Infinitely wise Creator has not left the creature without a power of moving the head a little in Aerial crustaceous animals, and the very eyes also in crustaceous Sea-animals; so that by these means they are inabled to direct some optick line or other against any object, and by that means they have the visive faculty as compleat as any Animal that can move its eyes.
Distances of Objects also, 'tis very likely they distinguish, partly by the consonant impressions made in some two convenient Pearls, one in each cluster; for, according as those congruous impressions affect, two Pearls neerer approach'd to each other, the neerer is the Object, and the farther they are distant, the more distant is the Object: partly also by the alteration of each Pearl, requisite to make the Sensation or Picture perfect; for 'tis impossible that the Pictures of two Objects, variously distant, can be perfectly painted, or made on the same Retina or bottom of the eye not altered, as will be very evident to any one that shall attentively consider the nature of refraction. Now, whether this alteration may be in the Figure of the Cornea, in the motion of access or recess of the Retina towards the Cornea, or in the alteration of a crustaline humour, if such there be, I pretend not to determine; though I think we need not doubt, but that there may be as much curiosity of contrivance and structure in every one of these Pearls, as in the eye of a Whale or Elephant, and the almighty's Fiat could as easily cause the existence of the one as the other; and as one day and a thousand years are the same with him, so may one eye and ten thousand.
This we may be sure of, that the filaments or sensative parts of the Retina must be most exceedingly curious and minute, since the whole Picture it self is such; what must needs the component parts be of that Retina, which distinguishes the part of an object's Picture that must be many millions of millions less then that in a man's eye? And how exceeding curious and subtile must the component parts of the medium that conveys light be, when we find the instrument made for its reception or refraction to be so exceedingly small? we may, I think, from this speculation be sufficiently discouraged from hoping to discover by any optick or other instrument the determinate bulk of the parts of the medium that conveys the pulse of light, since we find that there is not less accurateness shewn in the Figure and polish of those exceedingly minute lenticular surfaces, then in those more large and conspicuous surfaces of our own eyes. And yet can I not doubt, but that there is a determinate bulk of those parts, since I find them unable to enter between the parts of Mercury, which being in motion, must necessarily have pores, as I shall elsewhere shew, and here pass by, as being a digression.
As concerning the horns F F, the feelers or smellers, G G, the Probascis H H, and I, the hairs and brisles, KK, I shall indeavour to describe in the 42. Observation.