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moment; the fire sparkling like that of a forge to which the blast of a bellows is applied.

The image of an object when received through a convex lens will be inverted. If you cause the rays of light from the flame of a candle to pass through the glass of a common spectacle, and receive them on a sheet of paper, or dark skreen placed at a proper distance, you will see a complete inverted image of the candle on it. A convex lens placed in the hole of a window-shutter will exhibit, on a white sheet of paper situated in the focus of the glass, all the objects on the outside, as fields, trees, men, and houses, in an inverted order. The room should be quite dark, and the sun should shine upon the objects. A portable camera obscura may be made with a square box, in one side of which is to be fixed a tube, having a convex lens in it: within the box is a plane mirror, reclining backwards from the tube, in an angle of forty-five degrees. The picture is formed on a square of unpolished glass at the top of the box. If a piece of oiled paper be stretched on the glass, a landscape may be easily copied; or the outline may be sketched on the rough surface of the glass.

QUESTIONS.-1. What is a lens?-its axis?-focus? 2. Describe the five kinds of lenses. 3. What proportion is there between the common heat of the sun and the heat of the focus of a double convex lens? 4. Describe the burning glass formed at London. 5. What examples are given of images of objects being inverted by a convex lens? 6. How may a camera obscura be made? 7. Why is the mirror placed at an angle of 45 degrees exactly? Ans. To throw the image on the top, for incident rays, falling upon a surface declining 45 degrees, will be reflected at an equal angle of 45 degrees. 8. Describe figures 30. 36. 32. 33.

LESSON 33.

Mirrors.

Panoram'ic, exhibiting a succession of objects.

Opti'cian, a maker of optical instruments, one skilled in optics. MIRRORS are made of glass, silvered on one side, or of some metal highly polished. There are three kinds of them, the plane, the convex, and the concave. Objects seen in convex mirrors are diminished. A globe of glass, silvered

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on the inside, is sometimes suspended from the ceiling of a room. It affords a sort of panoramic view of surrounding objects, though not all in natural proportion of size. When a convex mirror can be placed before a window, either with a good prospect, or where there are a number of persons passing and repassing in their different employments, the images reflected from it will be erect, and behind the surface; and a landscape or a busy scene delineated on one of them is always a beautiful object to the eye. Concave mirrors make objects appear larger, but distorted. If one be hung on the wall of a room, or fixed in a chair, a person beyond the focus sees his image inverted. As he puts forward his hand the image in the glass appears to do the same, as if to shake hands. As he tries to clasp the hand it vanishes from his view. Let the spectator hold out a knife in his hand, the image will appear to do the same; and so strong will be the impression on his mind, that he will feel a reluctance to run his hand forward against the apparent weapon. A concave mirror throws back the sun's rays into one point or focus, where paper or gunpowder may be set on fire. Mirrors are sometimes made of a cylindrical concave form; and as one of them is placed either upright or on its side, the image of the picture is distorted into a very long or a very broad image. Reflecting surfaces may be made of various shapes, and if a regular figure be placed before an irregular reflector, the image will be deformed; but if an object, as a picture, be painted deformed, according to certain rules, the image will appear regular. Such figures and reflectors are sold by opticians, and they serve to astonish those who are ignorant of these subjects.

Small convex reflectors are made for the use of travellers, who, when fatigued by stretching the eye to Alps towering on Alps, can by their mirror, bring these sublime objects into a narrow compass, and gratify the sight by pictures which the art of man in vain attempts to imitate.

QUESTIONS.-1. What are the three kinds of mirrors? 2. How do convex mirrors make objects appear?-concave? 3. What are some of the experiments that may be performed with them? 4. How do cylindrical concave mirrors make an image of a picture appear? [NOTE. A mirror is sometimes called a Speculum, pl. Spec'ula.] 5. Describe fig. 27.

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Sem'icircle, a half round, part of a circle divided by the diameter.
Junct'ure, the line at which two things are joined together.
Prism, a solid piece of glass with three flat sides, and two equal
and parallel triangular ends.

SIR ISAAC NEWTON, to whom we are indebted for the most important discoveries respecting light and colours, was the first who divided a white ray of light, and found it to consist of an assemblage of coloured rays. This separation may be observed in the well known experiment of the prism. A ray being let into a darkened room, through a small round aperture in the shutter, and falling on a triangular glass prism, is, by the refraction of the prism, considerably dilated, and it will exhibit, on a skreen or on the opposite wall, an oblong image called a spectrum, variously coloured; the extremities of which are bounded by semicircles, and the sides are rectilinear. The colours are commonly divided into seven, which, however, have various shades gradually intermixing at their juncture. The following lines from Blackmore represent their order, beginning at the side of the refracting angle of the prism.

Of parent colours, first the flaming red

Sprung vivid forth; the tawny orange, next;
And next, delicious yellow; by whose side
Fell the kind beams of all-refreshing green;
Then the pure blue, that swells autumnal skies,
Ethereal played; and then, of sadder hue,
Emerged the deepened indigo, as when
The heavy skirted evening droops with frost,
While the last gleamings of refracted light
Died in the fainting violet away.

The union of these colours, in the proportions in which they appear in the spectrum, produce in us the idea of whiteness. If you paint a card in compartments with these seven colours, and whirl it rapidly on a pin, it will appear white. But a more decided proof of the composition of a white ray is afforded by uniting these coloured rays, and forming with them a ray of white light. This can be done

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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

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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 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.

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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