dulations or vibrations of a highly elastic medium, filling all space. Although the sun is the chief fountain or cause of light, it is produced in various ways independent of that luminary. Combustion produces *light, and so does the friction of various substances; it is also emitted from phosphori of different kinds.

Light is one of the most useful agents in nature, and one that particularly affects the animal and vegetable kingdoms. The action of light on vegetables causes them to give out oxygen gas, by which means the air becomes purified, and the vital principle restored which had been vitiated by respiration or other ways. Vegetables depend on light for their colour: a vegetable grows in the dark

it will be white and sickly, or etiolated, as it is termed. Gardeners blanch certain vegetables by depriving them of light; and such parts of vegetables as are shaded from the light, as the hearts of lettuces, of cabbages, &c., are naturally white.+ Vegetables in tropical regions flourish in a manner far superior to what they do with us; and discous vegetables, as the sun-flower, always turn towards the sun.

The colour of the animal creation as well as of the vegetable depends on light. In the polar regions, animals and birds are generally white, while they become more variegated as they approach the tropics. The inner feathers of birds, to which the air has no access, are generally white, and so are their bellies. The bellies of fish are white for the same reason. Man deprived of light soon droops, becomes sallow, dropsical, and dies.

If light consists of particles of matter, but this, notwithstanding the opinion of Sir Isaac Newton, is far from being generally entertained, they must be most minute, else, from the rapidity of their motion, they would seriously affect the sight. They, however, do not cause the slightest pain to the eye, nor when condensed by the most power

A piece of lime of the size of a pea acted on by an ignited stream of the mixed gases, oxygen and hydrogen, will produce a light equal to 120 wax candles, and with a reflector will cast a shadow ten miles distant. The most intense light is that produced by galvanism in an exhausted receiver.

There are some exceptions to this; in the ash and the elder the outer rind is pale coloured, while the inner bark is green.

ful burning glass, and thrown on the most delicate balance, do they affect it, although they would travel at the rate of nearly twelve millions of miles in a minute. In illustration of the smallness of the particles of light, supposing light to be a material body, if a candle be lighted and so placed that no object shall obstruct its rays, they will be diffused over a space of more than two miles in every direction, before the candle is apparently diminished in the slightest degree.

In reference to the velocity of light, it was thought for a long period that light was propagated instantaneously, but from observations by Roëmur on the eclipses of Jupiter's satellites, which regularly undergo occultations at stated periods, it was found that they were visible about sixteen minutes earlier when the earth was in that part of its orbit nearest Jupiter, than when in the opposite part, showing that light is about sixteen minutes in passing across the diameter of the earth's orbit, a distance of nearly 190 millions of miles. Circumstances of daily occurrence tend to illustrate the velocity of light. The flash of a gun is seen for a considerable time (if the distance is at all remote) before the report is heard; as also the lightning's flash before the thunder.

The rays of light always proceed in straight lines, except when they pass obliquely through media of different densities. Those rays of light from the sun that fall perpendicularly on the atmosphere, pass in a straight line to the earth, while those that fall obliquely undergo Refraction, or are bent as it were downwards out of their direct course. It is this refractive power of the atmosphere that causes twilight, and so gradually introduces in the morning the light of the sun, which, were it not for this, would blaze suddenly out from the thickest gloom; and at night, instead of quitting us so gradually and gently, would leave us all at once enveloped in darkness. Through the same cause the heavenly bodies appear higher in the heavens than they really are, and that in proportion to their proximity to the horizon; and when near the edge of the horizon they appear above it when they are in reality below it; so that when the lower limb of the sun or moon apparently just touches the horizon, although the whole

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body appears above it, it is in reality below it, as the upper limb is at that instant just beginning to touch the horizon. Rays of light become refracted in passing obliquely through water, glass, or any other diaphanous or transparent body, as well as through the atmosphere, and in passing from a rare to a denser medium, as from air into water, they are refracted towards a perpendicular to the surface; while on the contrary, in passing from a dense into a rarer medium, they are refracted farther from a perpendicular. If a stone be thrown obliquely into the water, when it strikes the water it will begin to fall more perpendicularly towards the bottom; just so is the direction of light in passing into water. The refractive nature of water may be seen by putting a stick partly into it, and observing the apparent change of the line of direction of the stick, both in and out of it: at the surface of the water it will appear broken. It is on the principle of the refractive power of light that the science of Dioptrics depends, which is the most useful and pleasing of all the sciences, since by its means the most remote objects are brought fully into our view, and the most minute are so magnified as to show every part of them distinctly.

When rays of light fall on an opaque body, through which they cannot pass, they either become absorbed by the body, or are reflected back again. It is by means of Reflection that bodies are rendered visible. Every body reflects particles of light, or vibrations, from every point of its surface, which move in straight lines and in all directions, and which by their action on the eye cause the body to be seen. The quantity of the rays of light reflected depends upon the nature of the body, the state and colour of the surface, and the angle of incidence. Under all these circumstances, the angle of reflection is equal to the angle of incidence. Of the various bodies which reflect light, metals possess this property in the greatest degree, and the lightest colours reflect the most. difference in the two extremes, white and black, are rendered evident by laying a slip of white cloth, and another of black on the snow in the sunshine; the black will be found to sink soon into the snow, by absorbing the light, and with it the heat; while the white piece will retain its place

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on the surface of the snow. While the science of Dioptrics depends on the refraction of light, and explains the construction of telescopes, microscopes, &c., that of Catoptrics depends on the reflection of light, and explains its laws and properties as reflected from mirrors or specula of all kinds, whether plain, concave, or convex.

It has been already observed that bodies are seen by the reflection of the rays of light from their surfaces; but the contrivance by which the idea of the body seen is conveyed to the mind, so as to form a correct judgment of it, is at once most wonderful and most effective. The Eye is the instrument for this purpose, and it is composed of such parts as best to answer the object for which it is designed.

Vision, or sight, is performed by rays of light reflected from the body seen passing through the humours of the eye, and painting the object on the retina at the back of the eye, whence it is conveyed by the optic nerve to the brain. The eye must for this purpose necessarily be of a convex figure, and of such a degree of convexity as the refractive powers of the several humours demand for forming the image at the given focal distance. The external part of the eye-ball is called the cornea or horny coat from its resemblance to a piece of transparent horn. Immediately behind this coat, there is a fine, clear humour, which, from its similarity to water, is called the aqueous humour. In this there is a membrane called the uvea, with a hole in the middle, termed the pupil, of a muscular texture, for altering the dimensions of the hole so as to regulate the quantity of light to be admitted. Behind this membrane is a substance of considerable consistency, and formed like a lens, called from its transparency the crystalline humour; this is contained in a fine tunic, called the choroides, and by means of a set of muscles may be moved a little nearer to the bottom of the eye or further from it to alter the focal distance, as necessity may require. The remaining part of the eye is filled with a jelly-like substance called the vitreous humour, and at the back is the retina, which is an expansion of the optic nerve, on which every thing that is seen is painted and conveyed by means of the optic nerve to the brain. The delineation of objects on the retina is prettily demonstrated by taking a bullock's

eye quite fresh, and, having stripped off the skin and the fat from the back part until only the thin membrane behind remains, placing it before any object, and the picture of the object will appear on the membrane, which is the retina; or the membrane may be taken away, and a piece of white paper substituted in its stead; the object will then appear on the paper, but in either case it will be inverted in the same manner as it would have been had the animal been alive. As the object seen is always painted in an inverted position on the retina, it very probably becomes again reverted as it ascends the optic nerve. It is most

wonderful that a prospect of many miles in extent, diversified with hill and dale, houses and fields, men and animals can be painted on so small a space as the retina of the eye. It may be observed that the magnitude of the object seen depends on the angle subtended by the object; consequently, were it not for the judgment, we should not have any correct idea of the relative size of bodies. As the angles under which bodies are seen, increase and decrease in a direct ratio, according to the distance, a man, fifty yards distant, is depicted on the retina twice as large as a man one hundred yards distant; and were it not for the judgment, the nearer might be supposed to be a man, and the more distant a little boy.

As only a very small part of an object can be seen distinctly at once, the eye is forced to turn itself successively to the different parts it is desirous of viewing. When an object is seen with both eyes, the axes of both eyes are directed to the object and meet there, and the optic nerves are so formed that the correspondent parts of each eye lead to the same spot in the brain, and produce but one sensation. If the axes of both eyes be not directed to the same object, the object will appear double, as the pictures do not fall on similar parts of the retina in both eyes. This may be perceived by holding the finger before any object, a lighted candle for instance, and by looking steadfastly at the object there will seem to be two fingers, and by looking at the finger the object will appear double. The

*It would be probably more correct to say four times, as he will appear twice as high, and twice as broad.