hem converge to a focus sooner, so as to paint
a distinct image on the retina.

The presbytical eye is remedied by a convex lens of proper convexity,
which makes the rays converge to a focus sooner, and thus causes
distinct vision: the sight of such persons is even more benefited by
a convex lens, than that of myopes by a concave one; for a convex
lens not only makes the picture of the object on the retina distinct,
but also more bright, by causing a greater quantity of light to enter
the pupil; while a concave one, at the same time that it renders
vision distinct, diminishes the quantity of light.

Long sighted persons commonly become more so as they advance in
years, owing to a waste of the humours of the eye; and even many
people whose sight was very good in their youth, cannot see without
spectacles when they grow old. The same waste in the humours of the
eye, is the reason why shortsighted persons commonly become less so
as they advance in years; so that many who were shortsighted in their
youth, come to see very distinctly when they grow old. Dr. Smith
seems to doubt this, and thinks that it is rather a hypothesis than a
matter of fact. I have however myself seen several instances in
confirmation of it; and it is very natural to suppose, that since
short and long sight depend upon directly opposite causes, and since
the latter is increased by age, the former must be diminished by it.

LECTURE IX.
THE LAWS OF ANIMAL LIFE.

In the preceding lectures I have taken a view, first of the general
structure and functions of the living body, and next of the different
organs called senses, by means of which we become acquainted with
external objects. I shall next endeavor to show that, through the
medium of these different senses, external objects affect us in a
still different manner, and by their different action, keep us alive:
for the human body is not an automaton; its life, and its different
actions, depend continually on impressions made upon it by external
objects. When the action of these ceases, either from their being
withdrawn, or from the organization necessary to perceive them, being
deranged or injured, the body becomes a piece of dead matter; becomes
obedient to the common laws of chemical attraction, and is decomposed
into its pristine elements, which, uniting with caloric, form gases;
which gases, being carried about in the atmosphere, or dissolved in
water, are absorbed by plants, and contribute to th