Book II

. ch. 9. § 8.)

This remarkable passage anticipates by implication the view of an interpretation of materials originally given in the visual sense, which, under the name of “suggestion,” is the ruling factor in the _New Theory of Vision_.

The following sentences relative to the invisibility of distances, contained in the _Treatise of Dioptrics_ (published in 1690) of Locke’s friend and correspondent William Molyneux, whose son was Berkeley’s pupil, illustrate Locke’s statements, and may be compared with the opening sections of the _Essay on Vision_:—

“In plain vision the estimate we make of the distance of objects (especially when so far removed that the interval between our two eyes bears no sensible proportion thereto, or when looked upon with one eye only) is rather the act of our judgment than of sense; and acquired by exercise, and a faculty of comparing, rather than natural. For, distance of itself is not to be perceived; for, ’tis a line (or a length) presented to our eye with its end toward us, which must therefore be only a point, and that is invisible. Wherefore distance is chiefly perceived by means of interjacent bodies, as by the earth, mountains, hills, fields, trees, houses, &c. Or by the estimate we make of the comparative magnitude of bodies, or of their faint colours, &c. These I say are the chief means of apprehending the distance of objects that are considerably remote. But as to nigh objects—to whose distance the interval of the eyes bears a sensible proportion—their distance is perceived by the turn of the eyes, or by the angle of the optic axes (_Gregorii Opt. Promot._ prop. 28). This was the opinion of the ancients, Alhazen, Vitellio, &c. And though the ingenious Jesuit Tacquet (_Opt. Lib. I._ prop. 2) disapprove thereof, and objects against it a new notion of Gassendus (of a man’s seeing only with one eye at a time one and the same object), yet this notion of Gassendus being absolutely false (as I could demonstrate were it not beside my present purpose), it makes nothing against this opinion.

“Wherefore, distance being only a line and not of itself perceivable, if an object were conveyed to the eye by one single ray only, there were no other means of judging of its distance but by some of those hinted before. Therefore when we estimate the distance of nigh objects, either we take the help of both eyes; or else we consider the pupil of one eye as having breadth, and receiving a parcel of rays from each radiating point. And, according to the various inclinations of the rays from one point on the various parts of the pupil, we make our estimate of the distance of the object. And therefore (as is said before), by one single eye we can only judge of the distance of such objects to whose distance the breadth of the pupil has a sensible proportion.... For, it is observed before (prop. 29, sec. 2, see also _Gregorii Opt. Promot._ prop. 29) that for viewing objects remote and nigh, there are requisite various conformations of the eye—the rays from nigh objects that fall on the eye diverging more than those from more remote objects.” (_Treatise of Dioptrics_,