to our principles, at least not rightly to agree with them. Nor is our
tenet alone struck at by this experiment, but likewise all others that
ever came to my knowledge are, every whit as much, endangered by it. The
ancient one especially (which is most commonly received, and comes
nearest to mine) seems to be so effectually overthrown thereby that the
most learned Tacquet has been forced to reject that principle, as false
and uncertain, on which alone he had built almost his whole CATOPTRICS;
and consequently by taking away the foundation, hath himself pulled down
the superstructure he had raised on it. Which, nevertheless, I do not
believe he would have done had he but considered the whole matter more
thoroughly, and examined the difficulty to the bottom. But as for me,
neither this nor any other difficulty shall have so great an influence on
me as to make me renounce that which I know to be manifestly agreeable to
reason: especially when, as it here falls out, the difficulty is founded
in the peculiar nature of a certain odd and particular case. For in the
present case something peculiar lies hid, which being involved in the
subtilty of nature will, perhaps, hardly be discovered till such time as
the manner of vision is more perfectly made known. Concerning which, I
must own, I have hitherto been able to find out nothing that has the
least show of PROBABILITY, not to mention CERTAINTY. I shall, therefore,
leave this knot to be untied by you, wishing you may have better success
in it than I have had.'

30. The ancient and received principle, which Dr. Barrow here mentions as
the main foundation of Tacquet's CATOPTRICS, is that: 'every visible point
seen by reflection from a speculum shall appear placed at the intersection
of the reflected ray, and the perpendicular of incidence:' which
intersection in the present case, happening to be behind the eye, it
greatly shakes the authority of that principle, whereon the
aforementioned author proceeds throughout his whole CATOPTRICS in