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 conneted with
various distances.
37 It plainly follows from what hath been said that a person perfectly
purblind (i.e. that could not see an object distinctly but when placed
close to his eye) would not make the same wrong judgment that others do
in the forementioned case. For to him greater confusions constantly
suggesting greater distances, he must, as he recedes from the glass and
the object grows more confused, judge it to be at a farther distance,
contrary to what they do who have had the perception of the objects
growing more confused connected with the idea of approach.
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