An Essay Towards a New Theory of Vision | Page 4

the
OBJECT do there make an ANGLE, by means of which, according as
it is greater or lesser, the OBJECT is perceived to be nearer or farther
off.
5. Betwixt which and the foregoing manner of estimating distance there
is this remarkable difference: that whereas there was no apparent,
necessary connection between small distance and a large and strong
appearance, or between great distance and a little and faint appearance,
there appears a very necessary connection between an obtuse angle and
near distance, and an acute angle and farther distance. It does not in the
least depend upon experience, but may be evidently known by anyone
before he had experienced it, that the nearer the concurrence of the
OPTIC AXES, the greater the ANGLE, and the remoter their
concurrence is, the lesser will be the ANGLE comprehended by them.
6. There is another way mentioned by optic writers, whereby they will
have us judge of those distances, in respect of which the breadth of the
PUPIL hath any sensible bigness: And that is the greater or lesser
divergency of the rays, which issuing from the visible point do fall on
the PUPIL, that point being judged nearest which is seen by most
diverging rays, and that remoter which is seen by less diverging rays:
and so on, the apparent distance still increasing, as the divergency of
the rays decreases, till at length it becomes infinite, when the rays that

fall on the PUPIL are to sense parallel. And after this manner it is said
we perceive distance when we look only with one eye.
7. In this case also it is plain we are not beholding to experience: it
being a certain, necessary truth that the nearer the direct rays falling on
the eye approach to a PARALLELISM, the farther off is the point of
their intersection, or the visible point from whence they flow.
8. I have here set down the common, current accounts that are given of
our perceiving near distances by sight, which, though they are
unquestionably received for true by MATHEMATICIANS, and
accordingly made use of by them in determining the apparent places of
OBJECTS, do, nevertheless seem to me very unsatisfactory: and that
for these following reasons:--
9. FIRST, It is evident that when the mind perceives any IDEA, not
immediately and of itself, it must be by the means of some other IDEA.
Thus, for instance, the passions which are in the mind of another are of
themselves to me invisible. I may nevertheless perceive them by sight,
though not immediately, yet by means of the colours they produce in
the countenance. We often see shame or fear in the looks of a man, by
perceiving the changes of his countenance to red or pale.
10. Moreover it is evident that no IDEA which is not itself perceived
can be the means of perceiving any other IDEA. If I do not perceive the
redness or paleness of a man's face themselves, it is impossible I should
perceive by them the passions which are in his mind.
11. Now from sect. 2 it is plain that distance is in its own nature
imperceptible, and yet it is perceived by sight. It remains, therefore,
that it be brought into view by means of some other IDEA that is itself
immediately perceived in the act of VISION.
12. But those LINES and ANGLES, by means whereof some
MATHEMATICIANS pretend to explain the perception of distance,
are themselves not at all perceived, nor are they in truth ever thought of
by those unskilful in optics. I appeal to anyone's experience whether
upon sight of an OBJECT he computes its distance by the bigness of
the ANGLE made by the meeting of the two OPTIC AXES? Or
whether he ever thinks of the greater or lesser divergency of the rays,
which arrive from any point to his PUPIL? Everyone is himself the best
judge of what he perceives, and what not. in vain shall all the
MATHEMATICIANS in the world tell me, that I perceive certain

LINES and ANGLES which introduce into my mind the various
IDEAS of DISTANCE, so long as I myself am conscious of no such
thing.
13. Since, therefore, those ANGLES and LINES are not themselves
perceived by sight, it follows from sect. 10 that the mind doth not by
them judge of the distance of OBJECTS.
14. Secondly, the truth of this assertion will be yet farther evident to
anyone that considers those LINES and ANGLES have no real
existence in nature, being only an HYPOTHESIS framed by the
MATHEMATICIANS, and by them introduced into OPTICS, that they
might treat of that science in a GEOMETRICAL way.
15. The third and last reason I shall give for rejecting that doctrine is,
that though we should grant