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 confusion
itself, without ever considering the cause from which it proceeds, doth
constantly annex the same degree of distance to the same degree of
confusion. Whether that confusion be occasioned by converging or by
diverging rays, it matters not. Whence it follows that the eye viewing
the object Z through the glass QS (which by refraction causeth the rays
ZQ, ZS, etc., to converge) should judge it to be at such a nearness at
which if it were placed it would radiate on the eye with rays diverging
to that degree as would produce the same confusion which is now produced
by converging rays, i.e. would cover a portion of the retina equal to DC
(VID. Fig. 3 supra). But then this must be understood (to use Dr.
Barrow's phrase) SECLUSIS PRAENOTIONIBUS ET PRAEJUDICIIS, in case we
abstract from all other circumstances of vision, such as the figure,
size, faintness, etc. of the visible objects; all which do ordinarily
concur to form our idea of distance, the mind having by frequent
experience observed their several sorts or degrees to be connected with
various distances.

37. It plainly follows from what hath been said that a person perfectl