Note dd — BOOK IV.

As Birds and Fishes are in divers Things conformable, so in some sort they are in their Eye; to enable it to correspond to all the Convergences, and Divergences of the Rays, which the Variations of each of the Mediums may produce. For this Service the Tunica Choroeides, (in Fishes) hath a musculous Substance at the Bottom of it, lying round the optick Nerve, at a small Distance from it; by which Means I imagine they are able to contract, and dilate the Choroeides, and thereby to lengthen and shorten the Eye: For the helping in which Service, I imagine it is that the Choroeides, and Sclerotica, are in a great Measure parted, that the Choroeides may have the greater Liberty of acting upon the Humours within.

But in Birds, I have my self found, that although the Choroeides be parted from the Sclerotica; yet the Choroeides hath no Muscle, but instead thereof, a curious pectinated Work, seated on the optick Nerve, represented in Fig. 2. In which c.a.e.b.d. represents the Choroeides and Sclerotica: a.b. the Part of the optick Nerve, that is within the Eye: v.v.v. the vitreous Humour: a.f.g.b. the Pecten: h.i. the Crystalline. For the Reception of this Pecten, the optick Nerve comes farther within the Eye, than in other Creatures. The Structure of this Pecten, is very like that of the Ligamentum Ciliare; and in the Eye of a Magpy, and some others, I could perceive it to be musculous towards the Bottom. This Pecten is so firmly fixed unto, or embodied in the vitreous Humour, that the vitreous Humour hangs firmly to it, and is not so easily parted from it. By which Means all the Motions of the Pecten are easily communicated to the vitreous Humour, and indeed to all contained in the Choroeides. And forasmuch as the Crystalline is connected to the vitreous Humour, therefore also the Alterations in the vitreous Humour affect also the Crystalline; and the Crystalline is hereby brought nearer unto, or farther from the Retina, as occasion is. Besides all which Observables in the Choroeides, and inner Eye, I have also found this farther remarkable in the Sclerotica, and outer-part of the Eye of Birds, viz. That the fore-part of the Sclerotica is horny and hard, the middle-part thin and flexible, and Braces intervene between the fore and hind-part, running between the Choroeides and Sclerotica; by which Means the Cornea, and back-part of the Eye, are brought to the same Conformity, that the rest of the Eye hath.

The great End and Design of this singular and curious Apparatus in the Eyes, both of Birds and Fishes, I take to be, 1. To enable those Creatures to see at all Distances, far off, or nigh; which (especially in the Waters) requireth a different Conformation of the Eye. In Birds also, this is of great Use, to enable them to see their Food at their Bill’s End, or to reach the utmost Distances their high Flights enable them to view; as to see over great Tracts of Sea or Land, whither they have occasion to fly; or to see their Food or Prey, even small Fishes in the Waters, and Birds, Worms, &c. on the Earth, when they sit upon Trees, high Rocks, or are hovering high in the Air. 2. To enable those Animals to adapt their Eye to all the various Refractions of their Medium. Even the Air it self varies the Refractions, according as it is rarer or denser, more or less compressed; as is manifest from the learned and ingenious Mr. Lowthorp’s Experiment in Phil. Trans. Nᵒ. 257. and some other Experiments since of the before-commended Mr. Hawksbee, both in natural, rarify’d and compressed Air; in each of which, the Refractions constantly varied in exact Proportion to the Rarity or Density of the Air. Vid. Hawksbee’s Exp. pag. 175, &c.

Besides this Conformity in general, between the Eyes of Birds and Fishes, Du Hamel tells us of a singular Conformity in the Cormorant’s Eye, and that is, that the Crystalline is globous, as in Fishes, to enable it to see and pursue its Prey under Water: Which J. Faber, in Mr. Willoughby saith, they do with wonderful Swiftness, and for a long Time. Will. Ornithol. p. 329.