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To explain a phrase which must presently be used, it may be remarked, that the term focus is Latin, and literally means a fire-place: it is therefore appropriately employed to denote that point where the rays of the sun are concentrated by a mirror; and hence when focal distance is mentioned, it will be at once intelligible.

The power of mirrors of a large diameter has been fully proved in later times. By these instruments the hardest metals, on which common fires, and even glasshouse furnaces, could produce no effect, have been melted in a few seconds. M. Villette, a Frenchman, nearly a century ago, constructed a mirror three feet eleven inches in diameter, and three feet two inches in focal distance, which melted copper ore in eight seconds, a fish's tooth in thirty-two seconds, iron ore in twentyfour seconds, cast iron in sixteen seconds, a silver sixpence in seven seconds, and tin in three seconds.

This mirror condensed the solar rays 17,257 times; a degree of heat which is about four hundred and ninety times greater than that of a common fire.

Mr. Parker of London also constructed a lens three feet in diameter, and six feet eight inches focus, which weighed 212 pounds. It melted twenty grains of gold in four seconds, and ten grains of platina in three seconds.

The broader the mirror or lens, and the shorter the focal distance, the more intense is the heat produced by such instruments. A globular decanter of water makes a powerful burning-glass; and household furniture has been set on fire by incautiously exposing it to the rays of the sun.

Count Buffon, too, formed a burning mirror of 168 small ones, each six inches square. By means of this instrument, with the faint rays of the sun in the month of March, he set on fire boards of beech wood at 150 feet distance. At another time he burnt wood at the distance of more than 120 feet, and silver was fused at 50 feet.

Any substance of a regular form which is used to reflect light, or to exhibit images of objects, is called a speculum or mirror. The latter name is commonly given to reflectors made of glass, which have quicksilver on the back, that they may reflect more light. The former name is applied to a metallic reflector.

A plane speculum or mirror, as A, has two plane surfaces, a b c d, parallel to each other. A double convex one shown at B, is a solid formed by two convex spherical surfaces, having their centres on opposite sides. A plane convex, represented at C, has one of its

surfaces convex and the other plane. A plane concave, described at D, has one surface concave and the other plane. A meniscus, which appears at E, has one surface convex and the other concave; the two meeting if continued. The same description applies equally to lenses. Mirrors were used in ancient times to produce illusion. An observer in front of a plane mirror sees a distinct image of himself; but if two persons take up a mirror, one standing as much on one side of a line perpendicular to the middle of it, as the other is on the other side, they will see each other, not themselves. This has been called the magician's mirror, and by some contrivances has been made to produce great surprise.

The wonder is, however, sometimes increased in other circumstances. A visitor to "the Gallery of Practical Science " may observe, for instance, at the end of the Long Room, what is called a "Phantom Box." He will see an opening, into which he may thrust his arm; but if he retires a few paces, he will

perceive a goldfinch sitting on a branch'; while this disappears as the aperture is approached. This singular effect is produced by a concave mirror at some distance behind, with this object placed before it in an inverted position. On this a strong light is cast, and the rays being prevented from reaching the opening, the figure is thrown into the aperture.

same means.

A still more striking effect has been produced by the The spectator sees, in this case, a basket of fruit, of which he is asked to partake, but as he approaches for that purpose, a person properly concealed, withdraws the real basket of fruit, and with the other hand advances a dagger, the image of which is not a little alarming.

These effects, known but

It was by such contrivances that superstition obtained great power in former times. to a few, served to lead in bondage the many. We read even of Hercules appearing in the ancient temple at Tyre, of Esculapius showing himself in another at Tarsus, and of a third edifice in Sicily, famous for exhibiting the goddesses to mortals. But in these instances optical illusions were no doubt used, and often in connexion with living objects.

There is reason to rejoice that the days of such

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deceptions have passed away. Many a sight, which has struck terror into the ignorant, has been shown to be as free from any just means of exciting alarm, as the white sign-post which has been conjured into a ghost. Many an object of wonder to the partially informed, has been easily explained by a larger amount of knowledge. "Within the last forty years," says Sir David Brewster, "science has brought these atmospherical spectres within the circle of her dominion; and not only are all their phenomena susceptible of distinct explanation, but we can even reproduce them on a small scale with the simplest elements of our optical apparatus."

But not only have we the light of Science; there is one unspeakably more valuable, the light of Divine truth. It was a distinguished privilege of the Apostle Peter, to behold the transfiguration of Christ, and to hear a voice from the excellent glory, saying: "This is my beloved Son, in whom I am well pleased." Matt. iii. 17. And this, it might be supposed, was not to be surpassed. But he distinctly declares that it is; and that in the experience of the peasant or the child who is the possessor of the Holy Scriptures. "We have," he says, "a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that