An Essay Towards a New Theory of Vision | Page 8

be
behind the eye, it greatly shakes the authority of that principle, whereon
the aforementioned author proceeds throughout his whole
CATOPTRICS in determining the apparent place of OBJECTS seen by
reflection from any kind of speculum.
31. Let us now see how this phenomenon agrees with our tenets. The

eye the nearer it is placed to the point B in the foregoing figures, the
more distinct is the appearance of the OBJECT; but as it recedes to O
the appearance grows more confused; and at P it sees the OBJECT yet
more confused; and so on till the eye being brought back to Z sees the
OBJECT in the greatest confusion of all. Wherefore by sect. 21 the
OBJECT should seem to approach the eye gradually as it recedes from
the point B, that is, at O it should (in consequence of the principle I
have laid down in the aforesaid section) seem nearer than it did at B,
and at P nearer than at 0, and at Q nearer than at P; and so on, till it
quite vanishes at Z. Which is the very matter of fact, as anyone that
pleases may easily satisfy himself by experiment.
32. This case is much the same as if we should suppose an Englishman
to meet a foreigner who used the same words with the English, but in a
direct contrary signification. The Englishman would not fail to make a
wrong judgment of the IDEAS annexed to those sounds in the mind of
him that used them. Just so, in the present case the OBJECT speaks (if I
may so say) with words that the eye is well acquainted with, that is,
confusions of appearance; but whereas heretofore the greater
confusions were always wont to signify nearer distances, they have in
this case a direct, contrary signification, being connected with the
greater distances. Whence it follows that the eye must unavoidably be
mistaken, since it will take the confusions in the sense it has been used
to, which is directly opposed to the true.
33. This phenomenon as it entirely subverts the opinion of those who
will have us judge of distance by lines and angles, on which
supposition it is altogether inexplicable, so it seems to me no small
confirmation of the truth of that principle whereby it is explained. But
in order co a more full explication of this point, and to show how far
the hypothesis of the mind's judging by the various divergency of rays
may be of use in determining the apparent place of an OBJECT, it will
be necessary to premise some few things, which are already well
known to those who have any skill in dioptrics.
34. FIRST, any radiating point is then distinctly seen when the rays
proceeding from it are, by the refractive power of the crystalline,
accurately reunited in the retina or fund of the eye: but if they are
reunited, either before they arrive at the retina, or after they have
passed it, then there is confused vision.

35. SECONDLY, suppose in the adjacent figures NP represent an eye
duly framed and retaining its natural figure. In Fig. 1 the rays falling
nearly parallel on the eye, are by the crystalline AB refracted, so as
their focus or point of union F falls exactly on the retina: but if the rays
fall sensibly diverging on the eye, as in Fig. 2, then their focus falls
beyond the retina: or if the rays are made to converge by the lens QS
before they come at the eye, as in Fig. 3, their focus F will fall before
the retina. In which two last cases it is evident from the foregoing
section that the appearance of the point Z is confused. And by how
much the greater is the convergency, or divergency, of the rays falling
on the pupil, by so much the farther will the point of their reunion be
from the retina, either before or behind it, and consequently the point Z
will appear by so much the more confused. And this, by the bye, may
show us the difference between confused and faint vision. Confused
vision is when the rays proceedings from each distinct point of the
OBJECT are not accurately recollected in one corresponding point on
the retina, but take up some space thereon, so that rays from different
points become mixed and confused together. This is opposed to a
distinct vision, and attends near objects. Faint vision is when by reason
of the distance of the object or grossness of the interjacent medium few
rays arrive from the object to the eye. This is opposed to vigorous or
clear vision, and attends remote objects. But to return.
36. The eye, or (to speak truly) the mind, perceiving only the