be gathering into a compact group; till at length, entering the bosom of the distant nebulæ, we should find ourselves surrounded by new and strange constellations; and if we saw our own firmament at all, should see it only as a faint annular nebula, far beyond the reach of all unassisted vision.

472. The great stellar cluster in which the sun and solar system are imbedded is supposed, in its form, to resemble a double convex lens, with the sun and solar system near its center; and by being viewed edgewise from our central position, to give us the phenomenon of the Milky Way.

The above is an edgewise view of the great stellar cluster, in the midst of which the solar system is placed, as drawn by Sir William Herschel. Its figure was ascertained by gauging the space-penetrating power of his telescope, and then "sounding the heavens, to ascertain the distance through the cluster, in all directions, to the open void. The nebulæ lie in distinct and independent islands, far beyond the limits of our cluster.

Let the student imagine the sun to be one of the stars near the middle of the lensshaped cluster, of which the above is an edge view, with the planets revolving close around it. If, then, he look out upon the surrounding stars, the number visible, and their distinctness, will depend upon the direction in which he looks. If toward the thin part of the cluster (either up or down in the cut), fewer stars will be seen, while they will be comparatively distinct. But if the view be toward the edge of the cluster, instead of the sides (or horizontally, in the cut), there will be seen beyond the large stars, and fading away to an indistinct and mingled light, a numberless host of stars; and this zone of distant stars will extend quite around the heavens. Such is the Galaxy or Milky Way. The zone of milky light is the light of the stars in the remote edge of the great cluster. The opening in the left end of the figure is a split in the cluster, and constitutes the division seen in the milky way, extending part way around the heavens. See cut page 203.

The vast apparent extent of the Galaxy, as compared with other nebulæ, is supposed to be justly attributable to its comparative nearness. Were we as far from the solar system as from the nebula in the Lyre, the Milky Way would doubtless appear as an annular nebula no larger than that. It may therefore with propriety be called "the great nebula of the solar system."

473. Sir W. Herschel estimated that 50,000 stars passed the field of his telescope, in the Milky Way, in a

472. Supposed form of our own stellar cluster? Philosophy of Galaxy? (Why apparently so large? How appear at a great distance?) 473. Stars in Milky Way? Mutual distances? Character of each star?

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single hour! And yet the space thus examined was hardly a point in the mighty concave of our own sunstrown firmament." What an idea is here conveyed to the mind, of the almost boundless extent of the universe! The mutual distances of these innumerable orbs are probably not less than the distance from our sun to the nearest fixed stars, while they are each the center of a distinct system of worlds, to which they dispense light and heat.

474. Were the universe limited to the Great Solar Cluster, in the midst of which we are placed, it would be impossible to conceive of its almost infinite dimensions; but when we reflect that this vast and glowing zone of suns is but one of thousands of such assemblages, which, from their remoteness, appear only as fleecy clouds hovering over the frontiers of space, we are absolutely overwhelmed and lost in the mighty abyss of being!

475. And here we close our rapid and necessarily imperfect survey of the Sidereal Heavens. And while the mind of the student is filled with awe, in contemplating the vastness and majesty of creation, let him not forget that over all these Jehovah reigns-that "these are but parts of his ways;" and yet so perfect is his knowledge and providence in every world, that the very hairs of our heads are numbered, and not a sparrow falls without his notice. And while we behold the wisdom, power, and goodness of God so gloriously inscribed in the heavens, let us learn to be humble and obedient-to love and serve our Maker here that we may be prepared for the still more extended scenes of another life, and for the society of the wise and good in a world to come.

474. Magnitude of our own cluster? What in comparison with all others? 475. Remarks in closing paragraph? Moral reflections?

PART III.

PRACTICAL ASTRONOMY.

CHAPTER I.

PROPERTIES OF LIGHT.

476. Practical Astronomy has respect to the means employed for the acquisition of astronomical knowledge. It includes the properties of light, the structure and use of instruments, and the processes of mathematical calculation.

In the present treatise, nothing further will be attempted than a mere introduction to practical astronomy. In a work designed for popular use, mathematical demonstrations would be out of place. Still, every student in astronomy should know how telescopes are made, upon what laws they depend for their power, and how they are used. It is for this purpose mainly that we add the following chapters on Practical Astronomy.

477. Light is that invisible ethereal substance by which we are apprised of the existence, forms, and colors of material objects, through the medium of the visual organs. To this subtile fluid we are especially indebted for our knowledge of those distant worlds that are the principal subjects of astronomical inquiry.

478. The term light is used in two different senses. It may signify either light itself, or the degree of light by which we are enabled to see objects distinctly. In this last sense, we put light in opposition to darkness. But

476. Parts of the book gone over? Subject of Part III.? Of Chapter I. ? What is practical astronomy? (How far discussed in this treatise ?) 477. Define light. For what indebted to it?

478. Different senses in which the term is used? What is darkness? Can it be dark and light at the same time? Is there any place without light? (Quotation from Dick?)

it should be borne in mind that darkness is merely the absence of that degree of light which is necessary to human vision; and when it is dark to us, it may be light to many of the lower animals. Indeed, there is more or less light even in the darkest night, and in the deepest dungeon.

"Those unfortunate individuals," says Dr. Dick, "who have been confined in the darkest dungeons, have declared, that though, on their first entrance, no object could be perceived, perhaps for a day or two, yet, in the course of time, as the pupils of their eyes expanded, they could readily perceive mice, rats, and other animals that infested their cells, and likewise the walls of their apartments; which shows that, even in such situations, light is present, and produces a certain degree of influence."

479. Of the nature of the substance we call light two theories have been advanced. The first is, that the whole sphere of the universe is filled with a subtile fluid, which receives from luminous bodies an agitation; so that, by its continued vibratory motion, we are enabled to perceive luminous bodies. This was the opinion of Descartes, Euler, Huygens, and Franklin.

The second theory is, that light consists of particles thrown off from luminous bodies, and actually proceeding through space. This is the doctrine of Newton, and of the British philosophers generally.

Without attempting to decide, in this place, upon the relative merits of these two hypotheses, we shall use those terms, for convenience sake, that indicate the actual passage of light from one body to another.

480. Light proceeds from luminous bodies in straight lines, and in all directions. It will not wind its way through a crooked passage, like sound; neither is it confined to a part of the circumference around it.

As the sun may be seen from every point in the solar system, and far hence into spaco in every direction, even till he appears but a faint and glimmering star, it is evident that he fills every part of this vast space with his beams. And the same might be said of every star in the firmament.

481. As vision depends not upon the existence of light merely, but requires a certain degree of light to emanate from the object, and to enter the pupil of the eye, it is obvious that if we can, by any means, concentrate the

479. What theories of the nature of light, and by whom supported respectively? (Remark of author?)

480. How light proceeds from luminous bodies? (Radiations from sun and stars?)

481. How improve vision, and why? (Animals?)

light, so that more may enter the eye, it will improve our perception of visible objects, and even enable us to see objects otherwise wholly invisible.

Some animals have the power of adapting their eyes to the existing degree of light. The cat, horse, &c., can see day or night; while the owl, that sees well in the night, sees poorly in the day-time.

482. Light may be turned out of its course either by reflection or refraction. It is reflected when it falls upon the highly polished surface of metals and other intransparent substances; and refracted when it passes through transparent substances of different densities.

REFRACTION OF LIGHT.

483. Whenever light passes from a rare medium to one more dense, and enters the latter obliquely, it invariably leaves its first direction, and assumes a new one. This change or bending of the rays of light is what is called Refraction.

The term refract is from the Latin re, and frango, to break; and signifies the break ing of the natural course of the rays.

484. As air and water are both transparent, but of different densities, it follows that, when light passes obliquely from one to

the other, it will be refracted. If it pass from the air into the water, it will be refracted toward a perpendicular.

D

AIR

LIGHT REFRACTED BY WATER.

WATER

Here the ray A C strikes the water perpendicularly, and passes directly through to B without being refracted. But the ray DC strikes the water at C obliquely; and instead of passing straight through to E, is refracte at C, and reaches the bottom of the water at F. If, therefore, a person were to receive the ray into the eye at F, and to judge of the place of the object from which the light emanates from the direction of the ray C F, he would conclude that he saw the object at G, unless he made allowance for the refraction of the light at C.

482. How light turned out of course?

B

F

E

483. What is refraction? How produced? (Derivation of term refract?) 484. How refracted by air and water? (Illustrate by diagram.)