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versity of colours which we so much admire, for the purpose of beautifying the scene, and rendering it a source of pleasureable enjoyment. It is an ornament which embellishes

nature wherever we behold her.

QUESTIONS.-1. Of what was Sir Isaac Newton the first discoverer? 2 How may a white ray of light be separated into the various colours of which it is composed? 3. How are the colours divided, and what are they called? 4. How is the idea of whiteness produced? What is the proof of this? 5. How may a substitute for a solid glass prism be made? 6. How is it proved that the colours which seem to proceed from coloured bodies do not belong to those bodies? 7. What are colours? 8. What is a spectrum? 9. Describe fig. 37.

LESSON 35.

The Rainbow, Halo, and Parhelia.

Parhe'lia, (singular, Parhe'lion) a bright light appearing on one side of the sun.

WHEN the rays of the sun strike upon drops of water falling from the clouds, and we are placed in such a direction that our back is towards the sun, and the clouds before us, we observe a peculiar phenomenon in the heavens, called a rainbow. We may consider the drops of rain as transparent globules upon which the rays fall, and are twice refracted and once reflected. Hence proceed the different colours of the rainbow. These colours appear the more vivid, as the clouds which are behind are darker, and the drops of rain fall closer. The drops continually forming produce a new rainbow every moment, and as each spectator observes it from a particular situation, it happens that scarcely two men strictly speaking, see the same rainbow; and this appear ance can only last whilst the drops which fall are succeeded by others.

Triumphal arch, that fill'st the sky

When storms prepare to part,

I ask not proud philosophy

To teach me what thou art

Still seen, as to my childhood's sight,
A midway station given

For happy spirits to alight

Betwixt the earth and heaven.

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Can all that optics teach, unfold
Thy form to please me so,
As when I dreamt of gems and gold
Hid in thy radiant bow.

When Science from Creation's face
Enchantment's veil withdraws
What lovely visions yield their place
To cold material laws.

And yet, fair bow, no fabling dreams,
But words of the Most High,
Have told why first thy robe of beams
Was woven in the sky.

When o'er the green undeluged earth
Heaven's covenant thou didst shine,
How came the world's grey fathers forth
To watch thy sacred sign!

And when its yellow lustre smiled
O'er mountains yet untrod,
Each mother held aloft her child
To bless the bow of God.

Methinks thy jubilee to keep,
The first-made anthem rang
On earth delivered from the deep,
And the first poet sang.

Nor ever shall the Muse's eye
Unraptured greet thy beam:
Theme of primeval prophecy,
Be still the poet's theme!

The earth to thee her incense yields,
The lark thy welcome sings,
When glittering in the freshen'd fields
The snowy mushroom springs.

How glorious is thy girdle cast
O'er mountain, tower, and town,

Or mirror'd in the ocean vast,
A thousand fathoms down.

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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

78

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 enIvelopes the eye, is called the choroid; 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

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called the aqueous humour, from its liquidity and resemblance to water. Beyond this is situated the crystalline it humour, so called from its clearness and transparency; 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 refrac tion 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 y 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.

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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

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