HALO AND PARHELIA.

As fresh in yon horizon dark,
As young thy beauties seem,
As when the eagle from the ark
First sported in thy beam.

For faithful to its sacred page,

Heaven still rebuilds thy span,

Nor lets the type grow pale with age,
That first spoke peace to man.

CAMPBELL.

77

In serene weather, we often observe a circular light, or luminous ring surrounding the moon; it is called a halo, or crown. Its outline sometimes faintly shows the colours of the rainbow. The moon is in the middle of this ring, and the intermediate space is generally darker than the rest of the sky. When the moon is at the full, and considerably elevated above the horizon, the ring appears most luminous. It is often very large. We are not right in supposing, that this circle really surrounds the moon; the true cause of such an appearance must be looked for in our atmosphere, the vapours of which make a refraction of the rays of light. False moons are sometimes seen near the real moon, and appear as large, but their light is paler. They are generally accompanied by circles, some of which have the same colours as the rainbow, whilst others are white, and others have long luminous tails. All these appearances are produced by refraction. The rays of light falling from the moon upon aqueous and sometimes frozen vapours, are refracted in various ways; the coloured rays are separated, and reaching the eye present a new image of the moon.

Parhelia or mock-suns are far more rarely seen, but their appearance is wonderfully curious. They generally appear about the size of the true sun, not quite so bright, though they are said sometimes to rival their parent luminary in splendour. When there are a number of them they are not equal to each other in brightness. Externally, they are tinged with colours like the rainbow. They are not always round, and have sometimes a long fiery tail opposite the sun, but are paler towards the extremity. They are formed by the reflection of the sun's beams on a cloud.

QUESTIONS.-1. Under what circumstances do we perceive the rainbow? 2. What is a halo? 3 What are parhelia, or mock-suns

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THE EYE.

LESSON 36.

Structure of the Eye.

Mem'branous, consisting of a web of several sorts of fibres interwoven together.

Op'tic, producing vision, subservient to vision.

Sclerotica, (pronounced skle-rot'-i-ca,) derived from a Greek word signifying hard.

THE body of the eye is of a spherical form. It has two membranous coverings; the external one is called the sclerotica; this has a projection in that part of the eye which is exposed to view, called the cor'nea, because, when dried, it has nearly the consistence of very fine horn, and is sufficiently transparent for the light to obtain free passage through it. The second membrane, which lines the cornea, and envelopes the eye, is called the chōroid; this has an opening in front just beneath the cornea, which forms the pupil, through which the rays of light pass into the eye. The pupil is surrounded by a circular border, which is a part of the choroid and called the iris, composed of a sort of network, which contracts or expands according to the force of the light in which it is placed. If a person sits looking towards a window, the pupils of his eyes appear very small, and the iris large. When he turns from the window, and covers his eyes with his hands, so as entirely to exclude the light for a few moments, the pupils will be enlarged and the iris diminished. This is the reason why the eyes suffer pain, when from darkness they suddenly come into a strong light; for the pupil being dilated, a quantity of rays must rush in before it has time to contract. And when we go from a strong light into obscurity, we at first imagine ourselves in total darkness; for a sufficient number of rays cannot gain admittance into the contracted pupil to enable us to distinguish objects: but in a few minutes it dilates, and we clearly perceive objects which were before invisible.

The choroid is imbued with a black liquor which serves to absorb all the rays that are irregularly reflected, and to convert the body of the eye into a more perfect camera obscura. Within these coverings of the eye-ball are contained three transparent substances, called humours. The first occupies the space immediately behind the cornea, and is

called the aqueous humour, from its liquidity and resemblance to water. Beyond this is situated the crystalline humour, so called from its clearness and transparency; it has the form of a lens, and refracts the rays of light in a greater degree of perfection than any that have been constructed by art. The back part of the eye, between the crystalline humour and the retina, is filled by the vitreous humour, which derives its name from its supposed resemblance to glass. The most important part of the eye is the retina; for it is that which receives the impression of the objects of sight, and conveys it to the mind. It consists of an expansion of the optic nerve of the most perfect whiteness: it proceeds from the brain, enters the eye and is finally spread over the interior surface of the choroid. The refraction occasioned by the several humours unites the whole of a pencil of rays, proceeding from any one point of an object, to a corresponding point on the retina, and the image is thus rendered distinct and strong. The muscles of the eye are six, and by the excellence of their arrangement it is enabled to move in all directions.

All three of the humours of the eye have some effect in refracting the rays of light, but the crystalline is the most powerful it is a complete double convex lens; and as every point of an object sends out rays in all directions, some rays from each point on the side next the eye will be converged and brought to as many points on the retina, and will form on it a distinct inverted picture of the object, which is seen erect by the habit of the mind. Although an image must be formed on the retina of each of our eyes, yet we do not see objects double; for when an object is seen distinctly with both eyes, the axis of each is directed to it, and the object appears single; but if the axes of both eyes are not directed to the object, it always appears double. If you look at any object, and then by pressing upon the under or upper side of one eye, remove it out of its natural place, you will see two objects, whose distance from each other will vary as the eye is more or less turned from its natural position.

It is well known that an object at a distance appears smaller than when it is near. The reason is, that the nearer any object can be brought to the eye, the larger will be the angle under which it appears; for the rays fall more diver

gent upon the crystalline humour, and consequently include a greater angle, and thus the object is magnified. In objects placed at such distances as we are used to, we know, by experience, how much an increase of distance will diminish their apparent magnitude, and we instantly suppose them of the size they would appear if they were less remote; but this can only be done, where we are well acquainted with the real magnitude of the object; in all other cases we judge of magnitudes by the angle under which the object appears at the known, or supposed distance; that is, we infer the real magnitude from the apparent magnitude in comparison with the distance of the object. Sight, therefore, does not represent extension such as it is in itself; it often deceives us both in regard to the size and the distance of objects, and we should be led into continual errors if experience did not set us right. This is rendered strikingly manifest from the case of a young man who was blind from his infancy, and who recovered his sight at the age of fourteen, by the operation of couching. At first he had no idea either of the size or distance of objects, but imagined that every thing he saw touched his eyes; and it was not till after having repeatedly felt them, and walked from one object to ⚫ another, that he acquired an idea of their respective dimen sions, their relative situations, and their distances.

QUESTIONS.-1. What is the external covering of the eye called?Describe it. 2. Describe the cornea. 3. The choroid. 4. The pupil. 5. The iris. 6. What is said in order to illustrate the contraction and dilatation of the iris? 7. Of what use is the black liquor in the choroid? 8. Describe the three humours of the eye. 9. Of what does the retina consist, and what is its use? 10. How is the image on the retina rendered distinct? 11. How does it appear that the image on the retina will be inverted? 12. Having two eyes, why do we not see objects double? 13. Why does a distant object appear smaller than one that is near? 14. How do we judge of the real magnitudes of objects? 15. What case is related to show that experience is necessary to correct the errors of sight? 16. Look at fig. 28. and describe the eye. [NOTE. Let the instructer explain to his pupils how objects of equal magnitudes appear under a greater angle when near, than when at a distance.]

SPECTACLES.

81

LESSON 37.

Optical Instruments.

Land'scape, the prospect of a country,—also a picture representing an extent of space with the various objects on it.

Glob'ule, a small particle of matter of a globular or spherical figure.

As the sight is the most noble and extensive of all our senses; as we make the most frequent use of our eyes in all the actions and concerns of life; that instrument which relieves the eyes when decayed, and supplies their defects, must be estimated as one of the greatest of advantages. Sight may be defective in various ways. Some eyes are too flat, others are too convex or found; in some, the humours lose a part of their transparency, and on that account, much of the light that enters the eye is stopped and lost in the passage, and every object appears dim. Spectacles are intended to collect the light and to bring it to a proper degree of convergency. The honour of their invention was claimed by Salvinus Armatus, a nobleman of Florence, who died in 1317, and the fact was inscribed on his tomb. When the eye is too flat, the rays proceeding from objects do not converge to a focus so soon as they reach the retina; in this case a convex glass is necessary, for it has the property of converging the rays, and of course, when suited to the eye, of bringing them to a focus, and forming an image on the retina. When the eye is too convex, the rays of light are converged to a focus before they reach the retina; to remedy this, a concave glass is used, which causes the rays to diverge, and prevents their coming to a focus too soon. Shortsighted persons bring objects close to their eyes; it has a similar effect to that produced by concave glasses; for the nearer an object is brought to the eye, the greater is the angle under which it is seen, that is, the extreme rays, and of course all the others, are made more divergent. But persons whose eyes are too flat, when examining an object, hold it at a distance, for the farther an object is held from their eyes, the less is the divergency of its rays, that is, the smaller is the angle under which it is seen the focal distance is increased, and an image is properly formed on the retina. In considering vision as achieved by the means of an image formed