ain. Dr. Porterfield thinks that the crystalline lens has a
motion by means of the ligamentum ciliare, by which the distance
between it and the retina is increased or diminished, according to
the different distances of objects. The ligamentum ciliare, he says,
is an organ, the structure and disposition of which excellently
qualify it for changing the situation of the crystalline, and
removing it to a greater distance from the retina, when objects are
too near for us; for that, when it contracts, it will not only draw
the crystalline forwards, but will also compress the vitreous humour,
lying behind it, so that it must press upon the crystalline, and push
it from the retina. Although this hypothesis will, in a great
measure, account for distinct vision at different distances, yet it
could only be of use where the rays enter the eye with a certain
degree of divergency, while, however we are sure, that in looking at
very distant objects which are at different distances from us, the
eye undergoes a change. But a sufficient objection to Dr.
Porterfield's hypothesis is, that it is by no means proved that the
crystalline lens can be moved in the manner he supposes, or that the
ligamentum ciliare is possessed of muscular fibres; on the contrary
some eminent anatomists deny that they are.

We shall now take a view of the opinion of M. de la Hire, who
considered this subject, as well as almost every other relating to
vision, with the closest attention; he maintains, that, in order to
view objects distinctly at different distances, there is no
alteration but in the size of the pupil, which is well known to
contract and dilate itself according to the quantity of light flowing
from the object we look at, being most contracted in the strongest
light, and most dilated when the light is weakest; and consequently
will contract when an object is held near the eye, and dilate as it
is removed, because in the first case the quantity of light entering
the eye is much greater than in the last. That this contraction of
the pupil will have the effect of rendering vision distinct,
especially when objects are within the furthest limits of distinct
vision, will plainly appear, if we consider the cause of indistinct
vision. Dr. Jurin has shown, that objects may be seen with sufficient
distinctness, though the pencils of rays issuing from the points of
them do not unite precisely in another point on the retina, but
instead thereof, if they form a