An Essay Towards a New Theory of Vision | Page 5

the real existence of those OPTIC
ANGLES, etc., and that it was possible for the mind to perceive them,
yet these principles would not be found sufficient to explain the
PHENOMENA of DISTANCE, as shall be shown hereafter.
16. Now, it being already shown that distance is suggested to the mind
by the mediation of some other IDEA which is itself perceived in the
act of seeing, it remains that we inquire what IDEAS or SENSATIONS
there be that attend VISION, unto which we may suppose the IDEAS
of distance are connected, and by which they are introduced into the
mind. And FIRST, it is certain by experience that when we look at a
near OBJECT with both eyes, according as it approaches or recedes
from us, we alter the disposition of our eyes, by lessening or widening
the interval between the PUPILS. This disposition or turn of the eyes is
attended with a sensation, which seems to me to be that which in this
case brings the IDEA of greater or lesser distance into the mind.
17. Not that there is any natural or necessary connection between the
sensation we perceive by the turn of the eyes and greater or lesser
distance, but because the mind has by constant EXPERIENCE found
the different sensations corresponding to the different dispositions of
the eyes to be attended each with a different degree of distance in the
OBJECT: there has grown an habitual or customary connection
between those two sorts of IDEAS, so that the mind no sooner
perceives the sensation arising from the different turn it gives the eyes,
In order to bring the PUPILS nearer or farther asunder, but it withal
perceives the different IDEA of distance which was wont to be

connected with that sensation; just as upon hearing a certain sound, the
IDEA is immediately suggested to the understanding which custom had
united with it.
18 Nor do I see how I can easily be mistaken in this matter. I know
evidently that distance is not perceived of itself. That by consequence it
must be perceived by means of some other IDEA which is immediately
perceived, and varies with the different degrees of distance. I know also
that the sensation arising from the turn of the eyes is of itself
immediately perceived, and various degrees thereof are connected with
different distances, which never fail to accompany them into my mind,
when I view an OBJECT distinctly with both eyes, whose distance is so
small that in respect of it the interval between the eyes has any
considerable magnitude.
19. I know it is a received opinion that by altering the disposition of the
eyes the mind perceives whether the angle of the OPTIC AXES is
made greater or lesser. And that accordingly by a kind of NATURAL
GEOMETRY it judges the point of their intersection to be nearer or
farther off. But that this is not true I am convinced by my own
experience, since I am not conscious that I make any such use of the
perception I have by the turn of my eyes. And for me to make those
judgments, and draw those conclusions from it, without knowing that I
do so, seems altogether incomprehensible.
20. From all which it follows that the judgment we make of the
distance of an OBJECT, viewed with both eyes, is entirely the
RESULT OF EXPERIENCE. If we had not constantly found certain
sensations arising from the various disposition of the eyes, attended
with certain degrees of distance, we should never make those sudden
judgments from them concerning the distance of OBJECTS; no more
than we would pretend to judge a man's thoughts by his pronouncing
words we had never heard before.
21. Secondly, an OBJECT placed at a certain distance from the eye, to
which the breadth of the PUPIL bears a considerable proportion, being
made to approach, is seen more confusedly: and the nearer it is brought
the more confused appearance it makes. And this being found
constantly to be so, there ariseth in the mind an habitual
CONNECTION between the several degrees of confusion and distance;
the greater confusion still implying the lesser distance, and the lesser

confusion the greater distance of the OBJECT.
22. This confused appearance of the OBJECT doth therefore seem to be
the MEDIUM whereby the mind judgeth of distance in those cases
wherein the most approved writers of optics will have it judge by the
different divergency with which the rays flowing from the radiating
point fall on the PUPIL. No man, I believe, will pretend to see or feel
those imaginary angles that the rays are supposed to form according to
their various inclinations on his eye. But he cannot choose seeing
whether the OBJECT appear more or less confused. It is