Read Ebook: The Practical Astronomer Comprising illustrations of light and colours--practical descriptions of all kinds of telescopes--the use of the equatorial-transit--circular and other astronomical instruments a particular account of the Earl of Rosse's large tel by Dick Thomas

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SECT. 3.--RAMAGE'S LARGE REFLECTING TELESCOPE.

Construction of this telescope, and the manner of using it--Illustrated by figures--Its properties and advantages--Tube not necessary in reflecting telescopes--How a large reflector might be constructed without a tube--How the form of a telescope may be used for viewing perspectives

SECT. 4.--EARL OF ROSSE'S REFLECTING TELESCOPES.

His mode of forming a large speculum, &c., see also, Appendix

SECT. 5.--REFLECTING TELESCOPES WITH GLASS SPECULA.

Various experiments on this subject, with their results

SECT. 6.--A REFLECTING TELESCOPE WITH A SINGLE MIRROR AND NO EYE-PIECE.

Experiments illustrative of this construction

ON THE EYE-PIECES OF TELESCOPES. ASTRONOMICAL EYE-PIECES.

TERRESTRIAL EYE-PIECES.

Eye-pieces with four lenses--Proportions of the focal lengths of these lenses--Dimensions and powers of several eye-pieces stated

DESCRIPTION OF AN EYE-PIECE, &C., OF AN OLD DUTCH ACHROMATIC TELESCOPE.

DESCRIPTION OF THE PANCRATIC EYE-TUBE.

MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES.

ON MEGALASCOPES, OR TELESCOPES FOR VIEWING VERY NEAR OBJECTS.

Mode of adapting a telescope for this purpose--objects to which they may be applied

REFLECTIONS ON LIGHT AND VISION, AND ON THE NATURE AND UTILITY OF TELESCOPES.

Wonderful and mysterious nature of light--The organ of vision, and its expansive range--Wonderful nature of the telescope, and the objects it has disclosed to view--No boundaries should be set to the discoveries of science and the improvement of art--The telescope is a machine which virtually transports us to the distant regions of space--It enlarges our views of the sublime scenes of creation--It has tended to amplify our conceptions of the empire and the attributes of the Deity--Various uses of this instrument in relation to science and common life

ON THE METHOD OF GRINDING AND POLISHING OPTICAL LENSES AND SPECULA.

ON VARIOUS ASTRONOMICAL INSTRUMENTS.

ON MICROMETERS.

ON THE EQUATORIAL TELESCOPE, OR PORTABLE OBSERVATORY.

Object of these observations--stars of the first and second magnitudes--General deductions from these observations

OBSERVATIONS ON THE PLANETS IN THE DAY-TIME.

OBSERVATIONS ON JUPITER AND OTHER PLANETS.

General conclusions, &c.

UTILITY OF CELESTIAL DAY OBSERVATIONS.

ON THE ASTRONOMICAL QUADRANT.

THE ASTRONOMICAL CIRCLE.

THE TRANSIT INSTRUMENT.

ON OBSERVATORIES.

Leading features of a spot adapted for celestial observations--Public and private observatories--Greenwich observatory--Instruments with which an observatory should be furnished--The Author's private observatory--Revolving domes for observatories--Cautions to be attended to in celestial observations

ON ORRERIES OR PLANETARIUMS.

DR. HENDERSON'S PLANETARIUM.

ON THE VARIOUS OPINIONS WHICH WERE ORIGINALLY FORMED OF SATURN'S RING, ILLUSTRATED WITH 13 VIEWS.

When and by whom its true figure was discovered.

ON THE SUPPOSED DIVISION OF THE EXTERIOR RING OF SATURN.

Kater's, Short's, Quetelet's and Decuppis's observations

Composition of the speculum, and the process of casting it--Mode of grinding and polishing it--Manner in which it is filled up--Expenses incurred in its construction--Results of observations which have been made with it--Two views representing this instrument and the buildings connected with it--Sir J. South's remarks and anticipations

LIST OF ENGRAVINGS.

THE

PRACTICAL ASTRONOMER.

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. 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.

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 description of such instruments, to take a cursory view of its nature and properties, in so far as they have been ascertained, and the effects it produces when obstructed by certain bodies, or when passing through different mediums.

GENERAL PROPERTIES OF LIGHT.

There is no branch of natural science more deserving of our study and investigation than that which relates to light--whether we consider its beautiful and extensive effects--the magnificence and grandeur of the objects it unfolds to view--the numerous and diversified phenomena it exhibits--the optical instruments which a knowledge of its properties has enabled us to construct--or the daily advantages we derive, as social beings, from its universal diffusion. If air, which serves as the medium of sound, and the vehicle of speech, enables us to carry on an interchange of thought and affection with our fellow-men; how much more extensively is that intercourse increased by light, which presents the images of our friends and other objects as it were immediately before us, in all their interesting forms and aspects--the speaking eye--the rosy cheeks--the benevolent smile, and the intellectual forehead! The eye, more susceptible of multifarious impressions than the other senses, 'takes in at once the landscape of the world,' and enables us to distinguish, in a moment, the shapes and forms of all its objects, their relative positions, the colours that adorn them, their diversified aspect, and the motions by which they are transported from one portion of space to another. Light, through the medium of the eye, not only unfolds to us the persons of others, in all their minute modifications and peculiarities, but exhibits us to ourselves. It presents to our own vision a faithful portrait of our peculiar features behind reflecting substances, without which property we should remain entirely ignorant of those traits of countenance which characterize us in the eyes of others.

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