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