HISTORY OF PHYSICAL ASTRONOMY.

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pose of employing it in physical observations of the celestial bodies. Atthe meeting of the British Association, which was held at Edinburgh in1850, it was agreed to memorialise the Government upon the advantageswhich would accrue to astronomical science from the practical adoption ofsuch a scheme. It is to be hoped that, ere long, steps will be takentowards its complete realisation.

CHAPTER XXL

Origin of Stellar Astronomy.—Physical Changes observed in the Starry Regions_

Disappearance of Stars from the Heavens.—New Stars.— Stars of Variable Lustre.—.Photometric Researches on the Stars. —Attempts to Determine their Apparent Dia*

meters. — Space-penetrating Power of Telescopes_Applied to ascertain the relative

Distances of the Stars. — Absolute Distances of the Stars determined by Photo-metric Principles.— Parallax of the Fixed Stars.—Early Researches on the Subject_

Modern Researches.—Bessel.—Henderson.—Struve..—Peters.—Proper Motions of

the Stars_Motion of the Solar System in Space..—Double Stars.—Discovery of their

Physical Connexion by Sir William Herschel. -—Methods for determining the Elementsof their Orbits.—Nebulae.—Speculations of Sir William Herschel. —Modern Re-searches on the Subject_Sir John Herschel. —The Earl of Rosse.—Early Specula-tions on the Milky Way..—Theory of Wright_Observations of Sir William Herschel .

— Speculations of that Astronomer on the breaking up of the Milky Way.—Re-searches of Struve on the Distribution of the Stars in Space—Gauges of Sir JohnHerschel in the Southern Hemisphere. —Speculations of M. Struve on the Extinctionof Light in its Passage through Space.

The aspect of the starry firmament furnishes one of the most gloriousspectacles which nature displays throughout the entire range of her widely-diversified empire. The multitude of luminaries with which the bluevault of heaven appears bespangled, the endless variety of lustre whichthey exhibit, the striking configurations which they form, and the calmregularity with which they pursue their nocturnal courses—all conspiretogether in awakening a feeling of adoration in the most listless spectator,while at the same time they are eminently calculated to excite theenquiries of the more rational observer of physical phenomena. Attentioncould not long have been directed to the appearance of the starry heavens,before it was discovered that, except in a few instances, the entire multi-tude of luminaries constantly maintained the same relative position.Hence they received the appellation of Fixed Stars , an expression, how-ever, which is now found to be inaccurate, since the researches of modernastronomers have served to demonstrate that many of the stars are affectedby very minute movements, while at the same time it can hardly bedoubted that they are all subject to a similar influence.

During the early period of astronomical science, the fixed stars wereobserved merely on account of their utility in forming reference points, bymeans of which the apparent positions of the planetary bodies might bedetermined with greater accuracy, and the laws of their movements henceascertained. Indeed, the Ptolemaic astronomy, by making the wholesphere of the stars perform a complete revolution round the earth oncein every twenty-four hours, assigned to those luminaries a very subordi-nate place in the physical universe. When the immortal Copernicus restored the true system of the world, the stars assumed their just dignityas vast bodies placed at an immeasurable distance from the earth; butit seemed hopeless ever to arrive at any knowledge respecting theirphysical constitution. At length the invention of the telescope, about