But it is plain it was
impossible for any man to attain to a right notion of this matter so long
as he had regard only to lines and angles, and did not apprehend the true
nature of vision, and how far it was of mathematical consideration.
40. Before we dismiss this subject, it is fit we take notice of a query
relating thereto, proposed by the ingenious Mr. Molyneux, is his TREATISE
OF DIOPTRICS,[Par. I. Prop. 31, Sect. 9.] where speaking of this
difficulty, he has these words: 'And so he (i.e. Dr. Barrow) leaves this
difficulty to the solution of others, which I (after so great an example)
shall do likewise; but with the resolution of the same admirable author of
not quitting the evident doarine which we have before laid down, for
determining the LOCUS OBJECTI, on account of being pressed by one
difficulty which seems inexplicable till a more intimate knowledge of the
visive faculty be obtained by mortals. In the meantime, I propose it to
the consideration of the ingenious, whether the LOCUS APPARENS of an
object placed as in this 9th section be not as much before the eye as the
distinct base is behind the eye!' To which query we may venture to answer
in the negative. For in the present case the rule for determining the
distance of the distinct base, or respective focus from the glass, is this:
as the difference between the distance of the object and focus is to the
focus or focal length, so the distance of the object from the glass is to
the distance of the respective focus or distinct base from the glass.
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