[aa] Thus Cats (their Pupils being erect, and the shutting of their Eye-lids transverse thereunto) can so close their Pupil, as to admit of, as it were, one only single Ray of Light; and by throwing all open, they can take in all the faintest Rays. Which is an incomparable Provision for these Animals, that have occasion to watch and way-lay their Prey both by Day and Night.
[bb] There is besides this large opening of the Pupil, in some nocturnal Animals, another admirable Provision, enabling them to catch their Prey in the Dark; and that is a Radiation of the Eyes: Of which Dr. Willis thus; Hujus usus est Oculi Pupillam, quasi jubare insito, illuminare, ut res noctu, & in tenebris positas conspicere valeat: quare in Fele plurimùm illustris est: at Homini, Avibus, & Piscibus deest. This Illumination he speaks of, is from the Tapetum, in the Bottom of the Eye, or the shining of the Retina, round the optick Nerve.
Besides which, he saith, the Iris hath a Faculty also, in some, of darting out Rays of Light, so as to enable them to see in the Dark: Of which he tells this Story; Novi quendam cerebro calidiori præditum, qui post uberiorem vini generosi potum in nocte atratâ, sive tenebris profundis, literas distincte legere potuit. Cujus ratio videtur esse, quòd spiritus animales velut accensi, adeòque ab hâc Iride irradiantes, jubare infito Medium illuminabant. Willis Ibid.
Such another Thing, Pliny tells us, was reported of Tiberius Cæsar: Ferunt Tib. Cæs. nec alii genitorum mortalium, fuisse naturam, ut expergefactus noctu paulisper, haud alio modo quam luce clarâ, contueretur omnia. Nat. Hist. l. 11. c. 37.
So Dr. Briggs: Virum sanè calidæ indolis novi in Comitatu Bedfordiensi degentem, qui oculis felineis——donatus est: adeò ut epistolam——mirè admodùm in loco obscuro (ubi eadem mihi vix apparuit) perlegit. Hujus verò Oculi (nisi quod Pupillas insigniores obsinuere) ab aliorum formatione neutiquam discrepabant. Ophthal. c. 5. §. 12.
[cc] The Tunica Aranea is taken notice of by Frier Bacon, who calls it, Tela Aranea, and saith, in hâc continetur——glaciale vel Crystallinum. Rog. Bacon’s Perspect. Distinct. 2. c. 3. The wrinkling of this, and the Cornea (as the Skin is of old Persons) he thinks is the Cause of the Obscurity of the Sight in such Persons. Bacon Ib. par. 2. cap. 2. But this Tunick some deny, and others allow of: Dr. A. M. of Trinity-College, Dublin, (in his Relat. of Anat. Obs. in the Eyes of Animals, in a Letter to Mr. Boyl, Ann. 1682. annexed to his Anat. Account of the Elephant burnt in Dublin, p. 57.) affirms the Tunica Aranea, and saith, I have often seen it before ’twas exposed to the Air one Minute, notwithstanding what Dr. Briggs saith to the contrary, &c. But Dr. Briggs his Opinion is, Humor Crystallinus, nisi aeri diutiùs expositus, vel lenitèr coctus (instar lactis) cuticulam non acquirit: quæ verò impropriè, Tunica Aranea dicitur, cùm si tantùm adventitia, ut in Oculo Bovis recens execto appareat. Briggs’s Ophthalm. c. 3.
The Crystalline Humour being of a double Substance, outwardly like a Gelly, towards the Center as consistent as hard Suet, upon occasion whereof its Figure may be varied; which Variation may be made by the Ligamentum Ciliare; Dr. Grew doth, upon these Accounts, not doubt to ascribe to the Ligamentum Ciliare, a Power of making the Crystalline more Convex, as well as of moving it to, or from the Retina. See Grew’s Cosmolog. Sacr. l. 1. c. 4. Now it is certain by the Laws of Opticks, that somewhat of this is absolutely necessary to distinct Vision, inasmuch as the Rays proceeding from nigh Objects do more diverge, and those from distant Objects less: Which requires either that the Crystalline Humour should be made more Convex, or more flat; or else an Elongation, or shortning of the Eye, or of the Distance between the Crystalline Humour and the Retina.
But although Dr. Briggs (so good a Judge) denies the Tunica Crystallina, contrary to the Opinion of most former Anatomists; yet there is great Reason to conclude he was in a Mistake, in my Opinion, from the Observations of the French Anatomists, of the Crystalline of the Eye, of the Gemp or Chamois, who say, The Membrana Arachnoïdes was very thick, and hard, so that it was easily separated from the Crystallinus, p. 145.
The same Anatomists also favour the Surmise of Dr. Grew, This [Contraction of the Fibres of the Ligamentum Ciliare on one side, and Dilatation on the other] would make us think that these Fibres of the Ligamentum Ciliare, are capable of Contraction, and voluntary Dilatation, like that of the Fibres of the Muscles; and that this Action may augment, or diminish the Convexity of the Crystallinus, according as the Need which the Distance of the Objects may make it to have on the Eye, to see more clearly and distinctly. Anat. Descrip. of a Bear, p. 49.
Since my penning the foregoing Notes, having as critically as I could, dissected many Eyes of Birds, Beasts and Fishes, I manifestly found the Membrana Arachnoïdes, and will undertake to shew it any one, with great Ease and Certainty. It is indeed so transparent, as not to be seen distinct from the Crystalline. But if the Cornea and Uvea be taken off before, or the vitreous Humour behind it, and the out-side of the Crystalline be gently cut, the Arachnoïdes may be seen to open, and the Crystalline will easily leap out, and part from the Ligamentum Ciliare; which otherwise it would not do: For it is by the Arachnoïdes braced to the Ligamentum Ciliare. This Membrane or Tunick, in the Ox, is so substantial and strong, though thin, that it yields to, or sinks under the sharpest Lancet, and requires (for so thin and weak a Membrane in appearance) a strong Pressure to pierce it.