A Treatise on the Eye/Book 2/Chapter 3
CHAP. III.
Of the Form of the Eye.
Sect.
Pag.
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| 1. | The Eye either spherical or spheroidal, | 65 |
| 2. | The Benefits arising from this Figure, | 66 |
CHAP. III.
Of the Form of the Eye.
Sect. 1. IN all Animals that I know, the Eye is of a round globular Form. In Man and Quadrupeds it is almost an exact Sphere; but in Birds and Fishes it is flat and depressed, both in its fore and back Part, and is rather spheroidal than spherical.
§ 2. This round Figure is of all others the most commodious for the Motion of this Organ; for all its Parts being nearly at the fame Distance from its Centre, none of them can strike against any Part of the Socket, or of the Fat which lines it, when the Eyes are moved; and therefore, by this Figure, our Eyes are well prepared to move every Way the Situation of Objects can require. And,
Secondly, By this Figure, our Eyes are best fitted to contain the Humours within, and to receive and refract the Rays of Light coming from Objects without, so as to form a Picture of them on the Bottom of our Eyes, for causing Vision. This scarce needs any Proof or Illustration; it being obvious, that, were it not for the Convexity of the Cornea, the Rays could not be made to converge so as to pass the Pupil, and form this Picture upon the Retina; as will appear more fully from what is to follow concerning the Manner of Vision and the Use of the several Parts of the Eye; which I shall not now anticipate, but shall only notice what the famous Fryar Bacon has said on this Head. His Words are: "Nam si esset planæ figuræ, species rei majoris oculo non posset cadere perpendiculariter super eum. Cum ergo oculus videt magna corpora, ut sere quartam Cæli uno aspectu, manisestum est, quod non potest esse planæ figuræ, nec alicujus nisi sphæricæ, quoniam super sphæram parvam possunt cadere perpendiculares infinitæ, quæ a magno corpore veniunt, et tendunt in centrum sphæræ: Et sic magmum corpus potest ab oculo parvo videri. For the Demonstration of this, he hath given us a Figure in his Perspectives, which I shall not now consider. But,
Thirdly, To these Reasons for the Sphericity of the Eye, I shall add another, which is seldom attended to by Authors, viz. That the Distance of the Retina or immediate Organ of Sight from the crystalline Humour, may be every where such as is necessary for receiving the Rays of Light which come from all the Points of the Object in their Focus. Every body knows, that there can be no distinct Vision, unless all the Rays which enter the Eye from the several Points of the Object be united by the Reflection of the Humours in so many distinct Points in the Bottom of the Eye; and therefore, was the Eye a Cube, or of any multangular Form, some Parts of the Retina would be too far off, and some too nigh, these refracting Humours, and so could not but receive the several Pencils of Rays, some before they meet in the Focus, and others after they meet, which would render the Images of Objects, and consequently Vision, very confused and indistinct: But, by means of the spherical Figure of this little Organ, the Humours are not only commodiously laid together for performing their Office of Refraction, but all the Parts of the Retina are placed at a due focal Distance behind the Chrystalline; which therefore must regularly receive the Images of Objects from without, and by transmitting them to the Sensorium, prevent that confused and indistinct Vision, which would necessarily happen, were the Eye of any other Figure.
But for a full Demonstration of this Point, I must refer to Opticians, who have demonstrated, 1mo, That if an Arch of a Circle described upon the Center of the Eye be looked at for an Object, its Image behind the Chrystalline will be a similar concentric Arch, whose Length will be to the Length of the Object in the Ratio of their Distances from the common Centre. 2do, If a streight Line cutting the Axis of Vision at right Angles be looked at for an Object, its Image behind the Chrystalline will be the Arch of an Ellipsis, whose Axis coincides with the Axis of the Eye. From these Propositions, which I must not stay to demonstrate, it is obvious, that when the visual Object is convex, its Image behind the Chrystalline will be an Arch whose Curvature must be greater than either that of a Circle or an Ellipsis. From all which, it is extremely plain, that Objects of all Figures are seen the more distinctly that the Eyes[errata 1] are spherical, and consequently that this Figure is the very best that could have been given to our Eyes.