any other cause it assumes too round a form, as in the case of short-sighted (or, as they are sometimes called, near-sighted) persons. Convex glasses are used when the eye is too flat, and concave glasses when it is too round.*

THE EYE.

233. The eye is composed of a number of coats, or coverings, within which are enclosed a lens, and certain humors, in the shape, and performing the office of con vex lenses.

1. The different parts of the eye, are:

* These lenses or glasses are generally numbered, by opticians, according to their degree of convexity or concavity; so that by knowing the number that fits the eye, the purchaser can generally be accommodated, without the trouble of trying many glasses.

It is the iris which gives the peculiar color to the eye.

What glasses are used when the eye is too flat? What are used when the eye is too round?

233. Of what is the eye composed? What are the different parts of the eye? First? Second? Third? Fourth? Fifth? Sixth? Seventh? Eighth? Ninth? Tenth? What does Fig. 112 represent? Explain the figure.

Its

same manner as the crystal of a watch is set in the case. degree of convexity varies in different individuals and in different periods of life. As it covers the pupil and the iris, it protects them from injury. Its principal office is to cause the light which reaches the eye to converge to the axis. Part of the light, however, is reflected by its finely polished surface, and causes the brilliancy of the eye.

5. The iris is so named from its being of different colors. It is a kind of circular curtain, placed in the front of the eye to regulate the quantity of light passing to the back part of the eye. It has a circular opening in the centre, which it involuntarily enlarges or diminishes.

6. The pupil is merely the opening in the iris, through which the light passes to the lens behind. It is always circular in the human eye, but in quadrupeds it is of different shape. When the pupil is expanded to its utmost extent, it is capable of admitting ten times the quantity of light that it does when most contracted.* In cats and other animals, which are said

* When we come from a dark place into a strong light, our eyes suffer pain, because the pupil being expanded, admits a larger quantity of light to rush in, before it has had time to contract. And when we go from a strong light into a faint one, we at first imagine ourselves in darkness, because the pupil is then contracted, and does not instantly expand.

What does Fig. 113 represent? Explain the figure. What part of the eye does the cornea form? Is its degree of convexity the same in all persons and all periods of life? What is its principal office? From what does the iris take its name? What is the use of the iris? What is the

pupil? What is its form in the human eye? How much more light is the pupil capable of admitting, when expanded to its utmost extent, thar when most contracted?

to see in the dark, the power of dilatation and contraction is much greater; it is computed, that their pupils may receive one hundred times more light at one time than at another. That light only, which passes the pupil, can be of use in vision; that which falls on the iris being reflected, returns through the cornea, and exhibits the color of the iris.

7. The aqueous humor is a fluid, as clear as the purest water. In shape it resembles a meniscus, and, being situated between the cornea and the crystalline lens, it assists in collecting and transmitting the rays of light from external objects to that lens.

8. The crystalline lens is a transparent body, in the form of a double convex lens, placed between the aqueous and vitreous humors. Its office is not only to collect the rays to a focus, on the retina, but also to increase the intensity of the light which is directed to the back part of the eye.

9. The vitreous humor (so called from its resemblance to melted glass) is a perfectly transparent mass, occupying the globe of the eye. Its shape is like a meniscus, the convexity of which greatly exceeds the concavity.

10. In Fig. 114 the shape of the aqueous and vitreous humors and the crystalline lens is presented. a is the aqueous humor, which is a meniscus, b the crystalline lens, which is a double convex lens, and c the vitreous humor, which is, also, a meniscus, whose concavity has a smaller radius than its convexity.

Fig. 114.

a

11. The retina is the seat of vision. The rays of light being refracted in their passage by the other parts of the eye, are brought to a focus in the retina, where an inverted image of the object is represented.

12. The choroid is the inner coat or covering of the eye. Its outer and inner surface is covered with a substance called the

What is said of those animals which are said to see in the dark? What light, only, is of use in vision? What becomes of the light which falls on the iris ? What is the aqueous humor? What is its form? Of what use is it? What is the crystalline lens? What is its office? What is the vitreous humor? Why do persons sometimes experience pain when passing from a dark place into strong light? What is the shape of the vitreous humor? Explain Fig 114 What is the retina? What is the choroid?

pigmentum nigrum, (or black paint.) Its office is, apparently, to absorb the rays of light immediately after they have fallen on the retina. It is the opinion of some philosophers, that it is the choroid and not the retina, which conveys the sensation produced by rays of light to the brain.

13. The sclerotica is the outer coat of the eye. It derives its name from its hardness. Its office is to preserve the globular figure of the eye, and defend its more delicate internal structure. To the sclerotica are attached the muscles which move the eye. It receives the cornea, which is inserted in it somewhat like a watch-glass in its case. It is pierced by the optic nerve, which, passing through it, expands over the inner surface of the choroid, and thus forms the retina.

14. The optic nerve is the organ which carries the impressions made by the rays of light, (whether by the medium of the retina, or the choroid,) to the brain, and thus produces the sensation of sight.*

234. The eye is a natural camera obscura,† and the images of all objects seen by the eye are represented on the retina, in the same manner as the forms of external objects are delineated in that instrument.

1. Fig. 115 represents only those parts of the eye which are most essential for the explanation of the phenomena of

* For the above description of the eye and its parts, the author is mainly indebted to Paxton's Introduction to the Study of Anatomy, edited by Dr. Lewis of this city.

+ The camera obscura is explained in a note on page 157.

By what is its outer and inner surface covered? What is its office? What is the opinion of some philosophers with regard to the choroid? What is the sclerotica? From what does it derive its name? What is its office? What are attached to the sclerotica? What is the optic nerve?

vision. The image is formed thus. The rays from the object cd, diverging towards the eye, enter the cornea c, and cross one another in their passage through the crystalline lens d, by which they are made to converge on the retina, where they form the inverted* image, ƒe.

2. The convexity of the crystalline humor is increased or

* Although the image is inverted on the retina, we see objects erect, because all the images formed on the retina have the same relative position which the objects themselves have; and as the rays all cross each other, the eye is directed upwards, to receive the rays which proceed from the upper part of an object, and downwards, to receive those which proceed from the lower part.

A distinct image is also formed on the retina of each eye; but as the optic nerves of the two eyes unite, or cross each other before they reach the brain, the impressions received by the two nerves are united, so that only one idea is excited, and objects are seen single. Although an object may be distinctly seen with only one eye, it has been calculated that the use of both eyes makes a difference of about one-twelfth. From the description now given of the eye, it may be seen what are the defects which are remedied by the use of concave and convex lenses, and how the use of these lenses remedies them. When the crystalline humor of the eye is too round, the rays of light which enter the eye converge to a focus before they reach the retina, and, therefore, the image will not be distinct; and when the crystalline humor is too flat, (as is often the case with old persons,) the rays will not converge on the retina, but tend to a point beyond it. A convex glass, by assisting the convergency of the crystalline lens, brings the rays to a focus on the retina, and produces distinct vision.

The eye is also subject to imperfection by reason of the humors losing their transparency, either by age or disease. For these imperfections no glasses offer a remedy without the aid of surgical skill. The operation of couching and removing cataracts from the eye, consists in making a puncture or incision through which the diseased part may escape. Its office is then supplied by a lens. If, however, the operator, by accident or want of skill, permit the vitreous humor to escape, the globe of the eye immediately diminishes in size, and total blindness is the inevitable result.

234. What philosophical instrument does the eye resemble in its construction? Explain Fig. 115. Note. Why do the objects appear erect when the images are inverted? Why do we see only one image when an image is formed on both eyes? What are the defects which are remedied by the use of concave and convex lenses? In what other way is the eye subject to imperfection? Is there any remedy for this?