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supposed to be the Pupil through which the Rays of Light are transmitted upon the Re-
tina. This Spot had three Circles surrounding it, and seemed seven Times less than the
Diameter of the whole Lens. He also numbered six thousand two hundred thirty-six
Pearls or Hemispheres in a Silkworm's two Eyes, when in the Fly State : three thousand
one hundred eighty-one in each Eye of a Beetle, and eight thousand in the two Eyes of
a common Fly.
The Author of Spectacle de la Nature sinely observes, | D? VIII.
of A? at P? p. 22. |
that the Eyes of other Creatures
are as it were multiplyed by Motion : whereas those of a Fly are fixed and immoveable,
and can only see what lies directly before them ; they are very numerous therefore, and
are placed in a round Surface, some in a high, others in a low Situation, to inform the
Fly of every Thing wherein she may be interested. She has a Number of Enemies,
but, with the Aid of these Eyes that surround her Head, she is enabled to discover what-
ever Danger threatens from above, behind, or on either Side, even when she is in full
Pursuit of a Prey directly before her.
These Eyes or little Hemispheres are placed, in all Kinds of Flies and aerial Animals, in
a most neat, regular, and admirable Ordination of triangular Rows, ranged as near to
one another as possible, and leaving the least Pits or Furrows between them that can
possibly be. But in Crabs, Lobsters, Shrimps, and such Kinds of crustaceous Water Ani-
mals, (whose Eyes are less pearled,) the Pearls are ranged in a quadrangular Order, the
Rows intersecting at right Angles, by which Disposition their Number on equal Surfaces
must be less : but to make them a Recompence for this, kind Nature has formed their
Eyes a little moveable, whereas those of flying Infects are all sixt.
The Goodness of Providence is particularly distinguishable in the Formation and Situa-
tion of the Eyes of different Animals, in a Manner most suitable to their different Ne-
cessities and Ways of Living. In Hares and Rabbets, whose Safety depends on slight,
they are very protuberant, and placed so much towards the Sides of their Heads, that
their two Eyes take in nearly a whole Sphere : whereas in Dogs (that pursue them) the
Eyes are set more forward in the Head, to look that Way more than backward.
In Cats the Pupil being erect, and the shutting of the Eye-Lids transverse thereto, they
can so close the Pupil, as to admit, as it were, one only single Ray of Light : and, on
the contrary, by throwing all open, they can take in all the faintest Rays. Which is an
incomparable Provision for Animals that have Occasion to watch and way-lay their Prey
both by Day and Night. But besides this, some nocturnal Creatures have a certain Ra-
diction or darting out of Rays of Light from their Eyes, enabling them to catch their Prey
in the Dark : and this most People have been Witnesses of in Cats.
Notwithstanding ARISTOTLE, PLINY, ALBERTUS MAGNUS, and several other
Writers were of Opinion that Moles are blind, the greater Diligence of the Moderns in
Dissections and Experiments have found them to have Eyes most excellently fitted for
their subterraneous Way of Life : not indeed much bigger than a large Pin's Head, but
which, it is supposed they have a Faculty of withdrawing, if not quite into the Head,
yet more or less within the Hair, as they have more or less Occasion to employ or
guard them | Derham's Phys. Theol, p. 94. |
.
The Eyes of Snails are placed at the Ends of their Horns, and are thrust out at some
Distance, or drawn quite within the Head as the Animal thinks proper.
Those of the Camelion turn backwards, or any Way else, like a Lens or convex Glass
in a versatile globular Socket, without any Motion of the Head; | Vid. Phil. Trans. No 137. Mem. for a Nat. Hist. |
and it is very extra-
ordinary to see one of the Eyes of this Creature moving, whilst the other remains fixt :
one turning forwards at the same Time the other looks behind, or perhaps one looking up
to the Sky, when the other turns itself downwards towards the Ground.
Several Opinions have prevailed amongst the Anatomists about the Reason why Man
having two Eyes sees not an Object double. GALEN and his Disciples thought this to
arise from a Coalition or Decussation of the optie Nerves ; but do not well agree whe-
ther they decussate, coalesce or only touch one another. The BARTHOLINES assert they
are united, | Bartholini Anat. lib. 3 c. 2. |
not simply by Contact, or Intersection, but by a total Confusion or Com-
mixture of their Substance. VESALIUS and some others have found a few Instances of
their being disunited, but say it is generally otherwise. Dr. GIBSON tells us, | Gibson's Anat. lib. 3. c. 10. |
they are
united by the closest Conjunction, but not Confusion of their Fibres. DES CARTES,
and some besides, judge this to be not from any Coalescence, Contact, or crossing of the
optie Nerves, but from a Sympathy between them. For, says DES CARTES, the Fibrillæ
constituting the medullary Part of those Nerves being spread in the Retina of each Eye,
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