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"But of this, and the manner of contracting of the Pupil, more, when I come to explain that part of the Eye; that which intention it for at present is, only to explain how the Eye becomes as it were a Hand, by which the Brain feels, and touches (the Objects, by creating a Motion in the Retina, the same, and at the same Instant; with the Motion of the lucid Object it self."

— Hooke, Robert (1635-1703)

Author

Hooke, Robert (1635-1703)

Work Title

Lectures of Light, Explicating its Nature, Properties, and Effects [from The Posthumous Works]

Place of Publication

London

Publisher

Printed by Sam. Smith and Benj. Walford

Date

Read 1680-1681, published 1705

Metaphor

"But of this, and the manner of contracting of the Pupil, more, when I come to explain that part of the Eye; that which intention it for at present is, only to explain how the Eye becomes as it were a Hand, by which the Brain feels, and touches (the Objects, by creating a Motion in the Retina, the same, and at the same Instant; with the Motion of the lucid Object it self."

Metaphor in Context

[...] Now by reason of the great Variety that there is in differing Objects, as to the Quantity and Strength, and that the Retina, or sensating part is capable only of receiving Impressions to a certain Degree of Strength, without being hurt by it, there is a contrivance in the Eye, which I shall afterwards more fully explain, by which the Quantity of the Rays admitted is moderated, so as to keep it, that the impression does not exceed that limit: And this is the Aperture or Hole through the Iris, which is the black Hole that appears in the middle of the transparent part of the Eye; for this Hole which admits the Rays to pass into the Eye, is contracted or dilated, according as the object is brighter or darker; that a lesser Quantity of the stronger Rays, and a greater Quantity of the weaker Rays may be admitted, and hence it is, that a brighter Object: among dimmer Objects, does cloud and darken them, because the aperture of the Iris being contracted protionable to the strength of the brighter, the Rays, admitted from the fainter and more dim Objects, are not sufficient to make a sensible Impression. So that the effect of the Rays are by this means proportion'd to the Ability of the sensible Part of the Eye to bear the Impression, and where notwithstanding the utmost contraction of this aperture of the Iris, the Rays make too strong an impression upon the Retina, we are forc'd to wink and close the Eye-Lids nearer, to shut out part of that quantity of Light which would otherwise have entred into the Eye; or to look through a small Hole, or through an opacous Body. And hence it is, that any one may with ease look upon the Sun, if he look through a small Pin-hole in a Plate, by which means one may with pleasure see an Eclipse of the Sun, without using any opacous Glass; though if the Cornea, or any other part of the Eye, be any ways opacous, this way discovers the Defects of them, and does somewhat vitiate the Figure of the Object. But of this, and the manner of contracting of the Pupil, more, when I come to explain that part of the Eye; that which intention it for at present is, only to explain how the Eye becomes as it were a Hand, by which the Brain feels, and touches (the Objects, by creating a Motion in the Retina, the same, and at the same Instant; with the Motion of the lucid Object it self. For the make of the Eye is such, in all its Contrivance and Parts, that the Conical Ray of Light proceeding from a Point of the Object, terminating with a Divergency in the Aperture, or Cornea of the Eye, ib Refraction thereof again reunited into a Point at the Focus, which is in the Retina [...]
(p. 124)