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.
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