by letting the coloured rays, which have been separated by a prism, fall upon a lens, which will converge them to a focus, and, being thus re-united, will appear white as they did before refraction.

Prisms are commonly made of solid glass, but those who do not possess one of this kind may easily make a substitute. Take three pieces of plate glass, each four or six inches long, and two or three inches wide; procure a tin frame, the two ends of which are in the exact shape of the three pieces of glass placed in the form of a triangle, with a strip of tin running from each angle of one end to the angles or corners of the other. These strips are bent so as to receive the two edges of the glass plates. The tin forming the ends is turned up so as to receive the plates, and one of the ends is furnished with a little tube to pour in water. When the frame and the glass plates are fastened together, and the crevices stopped, the prism is filled with clear water, and is ready for experiment.

When a spectrum is formed by the light which has passed through a prism upon a skreen, if a small hole be made through the skreen, and the rays of one colour only be permitted to pass through it, then whatever is viewed in that light, will appear of that particular colour. Thus if red light only has passed through the hole, then blood, or grass, or milk, viewed in that light behind the skreen, will appear red; excepting that the blood will appear of a stronger red colour than the grass or milk. If the blue light only has been transmitted through the hole, then the above mentioned substances will appear blue; and the like must be understood of the other colours. This proves that the colours, which seem to proceed from coloured bodies in general, do not belong to those bodies; but they are the component parts of the white light, in which those bodies are viewed, and that certain bodies have the property of absorbing some of those coloured rays of the white light which falls upon them, and of reflecting others. Thus, grass reflects the green rays and absorbs the rest; hence, the green rays coming to our eyes, render the appearance of grass green; thus blood absorbs every other coloured ray excepting the red, and so forth. Black bodies absorb all the seven coloured rays, and white bodies reflect them all. Providence appears to have decorated nature with the enchanting di

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 proyed 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.

Parhelia, (singular, Parheꞌlion) 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.

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

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,

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.

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 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 obWithin these coverings of the eye-ball are contained Three transparent substances, called humours. The first upies the space immediately behind the cornea, and is

scura.