The Practical Astronomer

PART I.
ON LIGHT.

INTRODUCTION.

Light is that invisible etherial matter which renders objects perceptible by the visual organs. It appears to be distributed throughout the immensity of the universe, and is essentially requisite to the enjoyment of every rank of perceptive existence. It is by the agency of this mysterious substance, that we become acquainted with the beauties and sublimities of the universe, and the wonderful operations of the Almighty Creator. Without its universal influence, an impenetrable veil would be thrown over the distant scenes of creation; the sun, the moon, the planets, and the starry orbs, would be shrouded in the deepest darkness, and the variegated surface of the globe on which we dwell, would be almost unnoticed and unknown. Creation would disappear, a mysterious gloom would surround the mind of every intelligence, all around would appear a dismal waste, and an undistinguished chaos. To whatever quarter we might turn, no form nor comeliness would be seen, and scarcely a trace of the perfections and agency of an All Wise and Almighty Being could be perceived throughout the universal gloom. In short, without the influence of light, no world could be inhabited, no animated being could subsist in the manner it now does, no knowledge could be acquired of the works of God, and happiness, even in the lowest degree, could scarcely be enjoyed by any organized intelligence.

We have never yet known what it is to live in a world deprived of this delightful visitant; for in the darkest night we enjoy a share of its beneficial agency, and even in the deepest dungeon its influence is not altogether unfelt.1 The blind, indeed, do not directly enjoy the advantages of light, but its influence is reflected upon them, and their knowledge is promoted through the medium of those who enjoy the use of their visual organs. Were all the inhabitants of the world deprived of their eye-sight, neither knowledge nor happiness, such as we now possess, could possibly be enjoyed.

There is nothing which so strikingly displays the beneficial and enlivening effects of light, as the dawn of a mild morning after a night of darkness and tempest. All appears gloom and desolation, in our terrestrial abode, till a faint light begins to whiten the eastern horizon. Every succeeding moment brings along with it something new and enlivening. The crescent of light towards the east, now expands its dimensions and rises upwards towards the cope of heaven; and objects, which a little before were immersed in the deepest gloom, begin to be clearly distinguished. At length the sun arises, and all nature is animated by his appearance; the magnificent scene of creation, which a little before was involved in obscurity, opens gradually to view, and every object around excites sentiments of wonder, delight, and adoration. The radiance which emanates from this luminary, displays before us a world strewed with blessings and embellished with the most beautiful attire. It unveils the lofty mountains and the forests with which they are crowned-the fruitful fields with the crops that cover them-the meadows, with the rivers which water and refresh them-the plains adorned with verdure, the placid lake and the expansive ocean. It removes the curtain of darkness from the abodes of men, and shows us the cities, towns and villages, the lofty domes, the glittering spires, and the palaces and temples with which the landscape is adorned. The flowers expand their buds and put forth their colours, the birds awake to melody, man goes forth to his labour, the sounds of human voices are heard, and all appears life and activity, as if a new world had emerged from the darkness of Chaos.

The whole of this splendid scene, which light produces, may be considered as a new creation, no less grand and beneficent than the first creation, when the command was issued, "Let there be light, and light was." The aurora and the rising sun cause the earth and all the objects which adorn its surface, to arise out of that profound darkness and apparent desolation which deprived us of the view of them, as if they had been no more. It may be affirmed, in full accordance with truth, that the efflux of light in the dawn of the morning, after a dark and cloudy night, is even more magnificent and exhilarating than at the first moment of its creation. At that period, there were no spectators on earth to admire its glorious effects; and no objects, such as we now behold, to be embellished with its radiance. The earth was a shapeless chaos, where no beauty or order could be perceived; the mountains had not reared their heads; the seas were not collected into their channels; no rivers rolled through the valleys, no verdure adorned the plains; the atmosphere was not raised on high to reflect the radiance, and no animated beings existed to diversify and enliven the scene. But now, when the dawning of the morning scatters the darkness of the night, it opens to view a scene of beauty and magnificence. The heavens are adorned with azure, the clouds are tinged with the most lively colours, the mountains and plains are clothed with verdure, and the whole of this lower creation stands forth arrayed with diversified scenes of beneficence and grandeur, while the contemplative eye looks round and wonders.

Such, then, are the important and beneficent effects of that light which every moment diffuses its blessings around us. It may justly be considered as one of the most essential substances connected with the system of the material universe, and which gives efficiency to all the other principles and arrangements of nature. Hence we are informed, in the sacred history, that light was the first production of the Almighty Creator, and the first born of created beings; for without it the universe would have presented nothing but an immense blank to all sentient existences. Hence, likewise, the Divine Being is metaphorically represented under the idea of light, as being the source of knowledge and felicity to all subordinate intelligences: "God is light, and in Him is no darkness at all;" and he is exhibited as "dwelling in light unapproachable and full of glory, whom no man hath seen or can see." In allusion to these circumstances, Milton, in his Paradise Lost, introduces the following beautiful apostrophe:-

As light is an element of so much importance and utility in the system of nature, so we find that arrangements have been made for its universal diffusion throughout all the worlds in the universe. The sun is one of the principal sources of light to this earth on which we dwell, and to all the other planetary bodies. And, in order that it may be equally distributed over every portion of the surfaces of these globes, to suit the exigencies of their inhabitants, they are endowed with a motion of rotation, by which every part of their surfaces is alternately turned towards the source of light; and when one hemisphere is deprived of the direct influence of the solar rays, its inhabitants derive a portion of light from luminaries in more distant regions, and have their views directed to other suns and systems dispersed, in countless numbers, throughout the remote spaces of the universe. Around several of the planets, satellites, or moons, have been arranged for the purpose of throwing light on their surfaces in the absence of the sun, while at the same time the primary planets themselves reflect an effulgence of light upon their satellites. All the stars which our unassisted vision can discern in the midnight sky, and the millions more which the telescope alone enables us to descry, must be considered as so many fountains of light, not merely to illuminate the voids of immensity, but to irradiate with their beams surrounding worlds with which they are more immediately connected, and to diffuse a general lustre throughout the amplitudes of infinite space. And, therefore, we have every reason to believe, that, could we fly, for thousands of years, with the swiftness of a seraph, through the spaces of immensity, we should never approach a region of absolute darkness, but should find ourselves, every moment encompassed with the emanations of light, and cheered with its benign influences. That Almighty Being who inhabiteth immensity and "dwells in light inaccessible," evidently appears to have diffused light over the remotest spaces of his creation, and to have thrown a radiance upon all the provinces of his wide and eternal empire, so that every intellectual being, wherever existing, may feel its beneficent effects, and be enabled, through its agency, to trace his wonderful operations, and the glorious attributes with which he is invested.

As the science of astronomy depends solely on the influence of light upon the organ of vision, which is the most noble and extensive of all our senses; and as the construction of telescopes and other astronomical instruments is founded upon our knowledge of the nature of light and the laws by which it operates-it is essentially requisite, before proceeding to a...