27. Thirdly, an OBJECT being placed at the distance above specified, and
brought nearer to the eye, we may nevertheless prevent, at least for some
time, the appearances growing more confused, by straining the eye. In
which case that sensation supplies the place of confused VISION in aiding
the mind to judge of the distance of the OBJECT; it being esteemed so
much the nearer by how much the effort or straining of the eye in order
to distinct VISION is greater.
28. I have here set down those sensations or IDEAS that seem to be the
constant and general occasions of introducing into the mind the different
IDEAS of near distance. It is true in most cases that divers other
circumstances contribute to frame our IDEA of distance, to wit, the
particular number, size, kind, etc., of the things seen. Concerning
which, as well as all other the forementioned occasions which suggest
distance, I shall only observe they have none of them, in their own
nature, any relation or connection with it: nor is it possible they should
ever signify the various degrees thereof, otherwise than as by EXPERIENCE
they have been found to be connected with them.
29. I shall proceed upon these principles to account for a phenomenon
which has hitherto strangely puzzled the writers of optics, and is so far
from being accounted for by any of their THEORIES OF VISION that it is,
by their own confession, plainly repugnant to them; and of consequence,
if nothing else could be objected, were alone sufficient to bring their
credit in question.
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