The whole difficulty I shall lay before you in the
words of the learned Dr. Barrow, with which he concludes his optic
lectures:--
'I have here delivered what my thoughts have suggested to me concerning
that part of optics which is more properly mathematical. As for the other
parts of that science (which being rather physical, do consequently
abound with plausible conjectures instead of certain principles), there
has in them scarce anything occurred to my observation different from what
has been already said by Kepler, Scheinerus, Descartes, and others. And
methinks, I had better say nothing at all, than repeat that which has
been so often said by others. I think it therefore high time to take my
leave of this subject: but before I quit it for good and all, the fair
and ingenuous dealing that I owe both to you and to truth obligeth me to
acquaint you with a certain untoward difficulty, which seems directly
opposite to the doctrine I have been hitherto inculcating, at least,
admits of no solution from it. In short it is this. Before the double
convex glass or concave speculum EBF, let the point A be placed at such a
distance that the rays proceeding from A, after refraction or reflection,
be brought to unite somewhere in the AxAB. And suppose the point of union
(i.e. the image of the point A, as hath been already set forth) to be Z;
between which and B, the vertex of the glass or speculum, conceive the
eye to be anywhere placed.
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