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ARAGO AND LARDNER'S ASTRONOMY.

PUBLISHER'S PREFACE.

TO ALL who are conversant with the existing state of Astronomical Science in Europe, it is well known, that, in addition to the regular duties of his office as Royal Astronomer of France, M. Arago has been in the practice of delivering each season at the Observatoire, a course of Lectures of a popular kind, which are attended by all classes of well informed persons, including ladies in considerable numbers. These discourses are given extemporaneously in the strictest sense of the term, and in style and character, bear a close analogy to those delivered by Dr. Lardner in this country within the last few years. It does not appear that M. Arago ever designed their publication, nor that he ever even committed them to writing. A person employed by one of the Brussels publishers, reported them, and the publication reputed to be M. Arago's Lectures, is nothing more than this report, which, though it could not be legally published or circulated in France, obtained through the Belgian booksellers, and their correspondents, an extensive illegal circulation in that country. A translation of this report was circulated largely in England.

The publishers of the present volume, being aware that errors of a more or less important kind, must, under such circumstances, have prevailed in the original Belgian edition, and still more in the English translation, and that omissions and chasms must have required to be filled up by some person conversant with the science, and capable of writing upon it in a clear and familiar style, applied to Dr. Lardner, and induced him to revise the reported Lectures, and to add to them such topics as might appear desirable to give them increased utility. The result of this arrangement has been the present volume.

Dr. Lardner desires it to be understood, that he should not have felt himself justified in interpolating any work, however elementary, published with the actual sanction of M. Arago's name. But, it being understood, and indeed manifested by unequivocal internal evidence, that the Belgian report was unathorized and unauthentic, and the circulation of some translation of it in this country being rendered inevitable, by the very popularity of its reputed author, it was better that a carefully revised copy should be published, than a mere reprint of the English translation of the imperfect Belgian report.

The paragraphs of this volume which are supplied by Dr. Lardner, are distinguished by asterisks.

LECTURE I.

OPTICAL INSTRUMENTS.

a distance, light would take a period of twentyfour thousand years! The importance of such an instrument must be obvious, and those who would comprehend the means by which astronomical science has attained its present state of advancement, must feel a corresponding desire to study and comprehend it.

*THE eye being the organ by which all as- || ment, that astronomer was enabled distinctly to tronomical knowledge is acquired, its powers, see individual stars, whose distance is two hunfunctions and structure, properly form the sub-dred times greater than that of the smallest star ject of this preliminary lecture. Although the || visible to the naked eye. To move over such natural range of the eye is vast, to a degree almost exceeding belief, yet the researches of modern astronomy have demanded a still wider scope. The distance of the smallest stars distinctly visible to the naked eye is known to be such, that light, which moves at the rate of nearly two hundred thousand miles per second, takes a hundred and twenty years to come from them to the earth. The unaided eye, therefore, gives us the survey of a sphere around us of that scarcely conceivable radius. But this is not enough. Art has supplied the telescope to extend still more widely the sphere of vision, and we can scarcely name any practical limits to the powers of this instrument. The largest and most powerful apparatus of this class, ever constructed, was the celebrated forty feet telesoope of Sir William Herschel. By this instru

GENERAL LAWS OF THE REFLECTION OF LIGHT

If we cause a ray of solar light to fall obliquely on a polished surface, we mark the following resulting phenomena:

1. A part of the light is reflected in a certain direction, and if the eye be placed somewhere in the line of that direction, it will perceive an image of the sun in the line of the reflected ray carried backward from the point of reflection.

2. The point where the incident ray meets

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