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It appears, too, that white is a due mixture of the seven primary colours wholly reflected; that black objects absorb all the rays, reflecting none; and that black is an effect of negation. Colours are but the phantoms of the dayWith that they're born, with that they fade away; Like beauty's charms, they but amuse the sight, Dark in themselves, till, by reflection bright, With the sun's aid, to rival him they boast;

But light withdrawn, in their own shades they're lost.

HUGHES,

588. Hence, white bodies in the sun are cool, and black ones hot; because white surfaces reflect all the light, and black ones absorb it: hence also, as red rays are the least refracted, they are supposed to be the largest, and are therefore painful to the eye; and hence, the most refrangible rays, as the smallest, are the most grateful.

Obs.When similar thermometers are placed in the different parts of the solar beam, separated by the prism, it is found, that different effects are produced in the dif ferent coloured raya. The greatest heat is exhibited in the red rays; the least in the violet rays; and in a space beyond the red rays, where there is no visible light, the increase of temperature is greatest of all. This important discovery was made by Dr. Herschel. He estimates the power of heating in the red rays, to be to that of the green rays as 55 to 26; and to that of the violet rays as 55 to 16. A thermometer, in the full red rays, indicated an increase of temperature of 7°. Fahrenheit, in ten minutes; beyond the red rays, in an equal time, the inerense was 9o. Fahrenheit. From these facts, it is evident, that matter set in motion by the sun, has the power of producing heat without light, and that its rays are Jess refrangible than the visible rays. Rays capable of producing heat, with and without light, proceed from bodies at the surface of the globe under peculiar agencies

or changes, as well as from the sun; and the phenomena, that are usually called the phenomena of the radiation of terrestrial heat, are of great extent and importance, and well worthy of being studied. There is another fact, still more extraordinary, which has been called the ra diation of cold, first observed by the Italian philosophers, and afterwards by Pictet. If in the arrangement of the two parallel mirrors, a piece of ice be introduced into the lower focus, the thermometer in the upper focus will indicate a diminution of temperature.

589. Rainbows arise from the rays of the sun which fall on drops of water being reflected and refracted to the eye of the spectator; and, of course, all those drops which are situated at the same angle all round from the eye, will present the same colour.

And, as different colours will arise at different angles; a bow composed of regular circles is a necessary consequence of showers of rain, while the sun shines.

It will, however, only be visible opposite to the sun; and the line from the sun through the eye of the spectator must be its centre.

590. As rays will reach the eye from drops of rain, owing to two different causes; so there will generally be two rainbows, one fainter, however, than the other.

The strongest, or lowermost rainbow, is ocoasioned by the light being reflected from the upper part of the back of the drops of rain; and the other, or upper bow, is occasioned by the light being twice refleeted within the drop from the lower part to the upper, and thence, refracted to the eye.

591. The breadth of a rainbow is about two

degrees; and, of course, no two spectators can see the same rays; but every eye will be the centre of its own bow.

All circles round the sun and moon arise in like manner, from the peculiar modifications of the rays passing through the vapours of the atmosphere in peculiar states.

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Obs. 1.The rays of light are shewn as passing from the sun to the drops of water, and thence to the eye of the spectator E; and all the rays at the same angle from the eye or centre P, necessarily produce the same colours.

Obs. 2.---The spectator stands, of course, with his back to the sun, and his eye is necessarily the centre of the hows, each drop of the same colour having an equal angle from the eye. Rainbows, of course, are more or less vivid, as the sun shines more or less bright on the opposite rain; and they are more or less perfect, as the rain is more or less diffused. An artificial bow may be made with a fountain; and glass chandeliers reflect colours on exactly the same principles.

Obs. 3. In the Inner Bow, the colours are red at top, then orange, yellow, green, blue, indigo, and violet and, in the upper Bow, the contrary. The upper Bow makes an angle with the eye of 54° and 51°; and the lower, of 49° and 40°. The centre of the circle is a line passing from the Sun, through the eye of the spectator.

592. A magic lanthorn is founded on the principle of placing the image within the focus of the lens; so that the rays diverge and produce a figure as much larger as is desired on a wall; and Phantasmagoria are produced by magic lanthorns; in which all the parts of the sliders, except the figures, are painted black and opake.

The camera obscura, for drawing landscapes, consists merely of one lens, with a mirror to reflect the images on the rough glass placed to draw upon with a pencil.

Obs. 1.-Before I leave this subject, I must recommend the tutor or student, to dissect a bullock's or sheep's eye. By taking off the back delicately, he will see the landscap before the eye beautifully painted on the optic nerve; he will find in front, the cornea; beneath it, the aqueous humour; then the iris; so called from its various colours; the pupil or hole; which opens and shuts to the light; and the crystalline humour, or lens. Then, the vitreous humour; and then, the optic nerve or net-work z ~a curious and wonderful arrangement!

2. The instantaneous motion of light has given rise to the TELEGRAPH, a modern invention of the greatest social importance, at present limited to purposes of govern ments; but capable of the highest uses to the community at large.

The telegraph consists of a large frame, in which are placed and worked six shutters, marked a, b, c, d, e, f, by means of ropes pulled in the manner of bell-ropes. By the various combinations of these shutters, 68 distinct signals may be produced, sufficient to represent the oi

I just 720 distinct aiguar.

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24 letters of the alphabet, the 10 digits, and various leading words. Such telegraphs are then set up on eminences, at the distance of 8, 10, or 12 miles; and a line of them, by repeating each other's signals, conveys a message from the first station to the last, at the rate of a hundred miles, in about five minutes!

The hole and telescope at T, is for the observer; who, in clear weather, is constantly on the look-out.

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XXVII. Meteorology,

593. Every 100 parts of the atmosphere is composed of 28 parts of oxygen, and 72 of azote or nitrogen, kept in a gaseous state by caloric or heat.

Atmospheric air is found to weigh in propor

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