The sight of myopes is remedied by a concave lens of proper concavity, which, by increasing the divergency of the rays, causes them to be united into a focus on the retina: and they do not require different glasses for different distances, for, if they have a lens which will make them see distinctly at the distance most commonly used by other persons, for example, at the distance at which persons whose eyes are good generally read, they will, by the help of the same glass, be able to see distinctly at all the distances at which good sighted people can see distinctly: for the cause of shortsightedness, is not a want of power to vary the conformation of the eye, but is owing to the whole quantity of refraction being too great for the distance of the retina from the cornea.
The other defect to be mentioned, is of an opposite nature, and persons labouring under it are called long sighted, or presbytae: it is caused by the cornea and crystalline, or either of them, being too flat in proportion to the distance between the crystalline and retina: whence it follows, that the rays which come from an object at an ordinary distance, will not be sufficiently refracted, and, consequently, will not meet at the retina, but beyond it, which will render the picture on the retina confused, and vision indistinct. Whence, in order to read, such persons are obliged to remove the book to a great distance, which lessens the divergency of the rays falling on the eye, and makes them 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 their nourishment. These are devoured by animals, which in their turn die, and are decompounded; thus, in the living world, as well as in the inanimate, every thing is subject to change, and to be renewed perpetually.
"Look nature through, 'tis revolution all,
All change, no death; day follows night; and night,
The dying day; stars rise, and set, and rise;
Earth takes th' example; see the summer gay,
With her green chaplet, and ambrosial flowers,
Droops into pallid autumn; winter gray,
Horrid with frost, and turbulent with storm,
Blows autumn and his golden fruits away,
Then melts into the spring; soft spring with breath
Favonian, from warm chambers of the south
Recals the first. All to reflourish, fades;
As in a wheel, all sinks to reascend."
The subject on which we are entering is of the utmost importance; for, by pointing out the manner in which life is supported and modified by the action of external powers, it discovers to us the true and only means of promoting health and longevity; for the action of these powers is generally within our own direction; and if the action of heat, food, air, and exercise, were properly regulated, we should have little to fear from the attacks of diseases.
When we examine the human body, the most curious and unaccountable circumstance that we observe, is its life, or its power of motion, sensation, and thought: for though the structure of the different parts which we have examined must excite our admiration and wonder, each part being admirably fitted for the performance of its different functions, yet without the breath of life, all these beautiful contrivances would have been useless. We have seen that the structure of the eye indicates in its contriver, the most consummate skill in optics; and of the ear the most perfect knowledge of sounds; yet if sensibility had not being given to the nerves which administer to these organs, the pulses of the air might have been communicated to the fluid in the labyrinth, and the rays of light might have formed images in the retina, without our being, in the smallest degree, conscious of their existence.
Though our efforts to discover the nature of life have hitherto been, and perhaps always will be, unsuccessful, yet we can, by a careful induction, or observation of facts, discover the laws by which it is governed, with respect to the action of external objects. This is what I shall now attempt to do.