The transmission of light is with about the velocity of 200,000 miles in a second. Light loses its intensity by diffusion; and this intensity is inversely proportional to the square of the distance.

The eye, the wonderful organ of vision, is a small globular body fitted with different coatings and humours adapted to the different purposes of sight; and placed in a cavity, in which it moves freely, to take in the different objects of the hemisphere.

The exterior coating is a white, opaque, horny substance called sclerotica. The front part of this coating, which is. perfectly transparent, is called the cornea. Within the sclerotica is the dark coloured coat, called the choroid. Behind the cornea the choroid is detached, and divided into two parts, one of which is in the shape of a ring, and forms the circular opening, called the pupil. The membrane, which forms this ring, is called the iris, on account of the variety of its colours. Behind the iris is a transparent body, shaped like a lens, dividing the interior of the eye into the anterior and posterior chambers. The lens shaped body is called the crystalline. The anterior chamber contains a transparent liquor, like water, called the aqueous humour. The posterior chamber is filled with a gelatinous substance, which is also transparent, and is called the vitreous humour. A line, which passes through the pupil, perpendicular to the two faces of the crystalline, is called the axis of the eye., When the axis varies from this line it produces squinting. Persons, who see well close at hand, are called myopes, or near sighted. Those, who see clearly at a distance, are called presbytes, or long-sighted.

It is an often vexed question, why it happens, that we see objects erect, when the painting of the image on the retina is inverted? This question turns upon a point, as little capable of solution, as the question, how the mind perceives objects? The image is probably presented erect to the mind by some inexplicable power of the imagination.

Plane mirrors reflect the objects that fall upon them. Glass mirrors are the well known instruments employed for

the common purposes of vision. But for exact optical experiments polished metallic mirrors should be used.

The axis of a concave mirror being directed towards the sun concentrates its rays to a focal point, producing intense heat. This is called a burning mirror. When we present the axis of a convex mirror towards the sun, it does not concentrate, it disperses the rays, and lessens the object reflected from its surface.

When a luminous ray passes obliquely into a transparent medium, it is refracted, that is, deflected towards the perpendicular, when the ray passes from a rare to a denser medium; and from it, when the ray passes from a dense into a rarer medium. When a ray passes from one medium into another, the sine of the incidence of the ray and of its refraction are in a constant ratio to each other.

The most important optical instruments are lenses of different forms and dimensions, to answer different intentions. The common convex lens, or burning glass, converges the sun's rays to a focal point; and its effects are as remarkable, in producing intensity of heat, as the concave mirror. Common spectacles are generally glasses of this character. A lens of this description, whose focal distance is less than half an inch, is called a microscope. There are microscopes, that magnify an object more than half a million times. The solar microscope is most commonly used for the purposes of microscopic observation, having the advantage of enabling a number of persons to see the object at the same moment.

In the camera obscura the images of distant objects are formed by a converging glass thrown upon some surface. These images can be reflected upward or downward by a plane mirror, placed at some distance behind the glass at an angle of 45°; so that the images may be represented upon a horizontal plane. This is a most important instrument for a landscape painter, as the landscapes are represented with great beauty, and in perfect accuracy of proportion.

The camera lucida consists of a quadrangular base, before which is placed a convex lens of considerable extent.

Behind this in the base is a plane mirror, placed at an angle of 45°, which reflects towards the cover the images of distant objects. This is also much used by landscape painters.

By means of two large convex glasses, placed at a little distance from each other, and by passing pictures before them painted upon glass, all the beautiful experiments, at once so fascinating and inexplicable to untaught beholders, such, for example, as the phantasmagoria and spectral appearances, of the magic lantern are produced.

A tube fitted with lenses, for the purpose of viewing objects at a great distance, is called a telescope. Herschell's telescope possesses a very great magnifying power. The telescope of Dolland is called acromatic, or colorless, because it exhibits the object without any of the prismatic colors. The making of telescopes, capable of answering all astronomical purposes with entire accuracy, is a work of great difficulty, principally owing to the different refracting powers of the various lenses employed, and to the want of perfect uniformity in the substance of any glass, that has been yet made.

While Newton was settling the courses of the stars, and decomposing light, Buffon laid open the wonders of the irrational animate creation, and placed the living universe in review before our eyes. The superb courser, kindling at the sound of the trumpet, and arching his neck 'clothed with thunder,' the lion, the tyrant monarch of the burning African plains, the tiger, ranging over the arid sands, thirsting for blood, the fleet stag, the beavers constructing their city with mathematical precision, and the profoundest knowledge of hydraulics, in the peaceful waters, the soaring eagle, the splendid humming bird, all live in his graphic descriptions. Living nature seemed to pass before him, as before our first father, to be classed, and named. As he gave the names, he developed the instincts and dispositions of the animals, and taught us, if I may so say, their moral character. The eloquent painter of nature aspired to show us, how the system of the universe was reared. According to him the Creator hurled an immense comet upon the sun, which struck off a glittering and red hot spark from that planet,

which, in process of time, became our globe. He supposes it whirling in space, in which it kindled an immense conflagration, for three thousand years. Such a proportion of the waters, the while, were reduced to vapor, as to cause the dry land of the continents to appear. To people the world with its variety of living beings, he has recourse to a theory of organic molecules, not unlike the atomic philosophy of Epicurus. His imposing eloquence has indeed thrown an attractive brilliancy over his visionary system. But no effort is necessary to crush his colossal image of imaginations, like that of Dura, partly of gold and partly of clay. His philosophic reveries have already passed into oblivion. His zoology, on the contrary, will live forever. He faithfully observed living nature, and has produced in his writing those strong and faithful pictures, which cannot be mistaken.

The illustrious Swedish naturalist Linneus came after him, emulating his march. Patient, industrious, and highly endowed, he has classed the plants and flowers from the cedar of the mountains to the hydrangea of the brook.

A man equally illustrious in his own walk arose after them. Lavoisier devoted himself to the investigation of matters, simple and unimportant in appearance, which had been overlooked by preceding philosophers. He became the founder of chemistry in such a sense, as almost to create a new science, which science seems almost endowed with the power to create. He decomposed, and recomposed water and air; and the age was taught to recognize new elements in substances, which had hitherto been considered simple and elementary. The maker of porcelain came to him for his earths, the painter for his colors, the physician for his remedies, and the warrior for his steel. All the other sciences drew resources from the new fountains which he opened. He created mineralogy in France, reformed geology, and penetrating the earth, taught us many of the secrets, that nature had hitherto concealed in her bosom. To his researches we owe much of the exactness of our knowledge respecting our food, dress and arts; our gold, silver and iron, of gunpowder and the mineral medicines.

It would be difficult to find limits to the utility of his discoveries. Contemplating the wonderful labours of this great man, of Sir Humphry Davy, and a host of chemists, who have followed in their steps, we might imagine, that this new science was exhausted.

But it is only a little portion of the surface of our sphere, about which we have any knowledge. Of its depths and centre we remain wholly ignorant. A few scattered rays of the light of truth, which glimmer here and there in the general darkness, have been discovered. The true philosophers, like fugitive points, collect, and diffuse, from time to time, a little light. To inspire the learned with modesty, it is a fact worthy of remark, that the most important discoveries have been made by persons, who laid no claims to the reputation of learning. The discovery of gunpowder was not made by professed chemists, but by a poor monk. The hint for the first perspective glasses was not given by opticians, but by children in their sports. The discovery of America was not due to the proud priests and geographers of the king of Spain, but to an obscure Genoese pilot, who sought a western route to the East Indies and the spice islands. The discoverer of Herschell's new planetary satellites, and a vast number of the celestial phenomena, was not an astronomer by original profession, but a musician of the royal guards of the British king. The discoverer of the identity of lightning and electricity, the illustrious Franklin, commenced his career not in academic shades, or under the shelter of great names, but in the obscure garret of a printer. The name of Jenner would have been lost in obscurity, but for the immortal discovery of the vaccine disease; and Fulton has left no heritage to his posterity, but honorable poverty, and the renown of having first successfully applied steam to the driving of vessels upon rivers. That would be a useful book, which should indicate all the discoveries, made by chance or by the unlearned.