358
HISTORY OF PHYSICAL ASTRONOMY.
dimensions, the diffraction of the ohject-glass begins to affect the apparentmagnitude of the image in the opposite direction, so that when the aper-ture has reached a certain degree of contraction, the enlargement arisingfrom this cause may compensate for the diminished effect of irradia-tion. Prof. Powell is of opinion that this may account for Dr. Kobinsonhaving found the interval between the transits of the sun’s limbs to beunaltered by contracting the aperture of the telescope, and also for thefact that Mr. Dawes, during the transit of Mercury over the sun’s diskin 1848, found that the apparent diameter of the planet was the same,when observed with different apertures.
A review of the progress of researches on irradiation cannot fail to sug-gest the conclusion, that the knowledge we possess respecting its natureand laws is still very imperfect and obscure. On the other hand, no doubtcan exist that it exercises a sensible influence on the observations of theastronomer. It is manifest also, that as the art of observation continuesto advance towards perfection, it will become more and more imperativeon the part of the astronomer to attend to the effects produced by it. Inthe present state of the subject the inquirer is impeded at the very outsetof his researches, by the want of data of sufficient completeness and pre-cision upon which he might establish his reasoning. It is to be hopedthat amid the activity which pervades every department of astronomicalscience, this branch of enquiry will not fail to receive an amount of atten-tion adequate to its growing importance.
We shall now proceed to give a brief account of the researches of astro-nomers, with a view to the explanation of various interesting phenomena,by means of some of the principles alluded to in this chapter.
CHAPTER XVII.
Eclipses of the Sun and Moon_Historical Statement of total Eclipses of the Sun.—
Annular Eclipses observed in modern times.—Change of Colour which the sky under-goes during an Eclipse.—Its explanation by M. Arago.—Corona of Light observedaround the Moon.—Allusions made to it by Ancient Authors.—Explanations of itsPhysical Cause bv different Individuals.—Protuberances on the Moon ’s Limb.—Theirmost probable nature.—Observations on the surface of the Moon during Eclipses—Undulations observed on the occasion of the Eclipse of 1842.—Similar Phenomenaobserved during the Eclipse of 1733.—Explanation of their Origin.—OpticalPhenomena observed during Solar Eclipses.—Threads, Beads, &c.—Explanation of
their Origin—Lunar Eclipses Transits of Venus.—Physical appearances observed
during their Occurrence.—Transits of Mercury.—Spot observed on the Planet’s disk.—Its explanation by Professor Powell.—Occultations of the Planets and Stars.
Amid the variety of grand and beautiful phenomena which flow fromthe operation of nature’s laws, exciting alternately the admiration anddelight of the attentive observer, there are some whose surpassing magni-ficence extorts universal homage, and awakens in the spectator a feeling ofreverential awe. It is impossible for the most frivolous mind to regardwith indifference the irresistible impetuosity of the hurricane as in itswild career it sweeps along every opposing obstacle, or to contemplatewithout emotion the sublime spectacle of the ocean when its billows areagitated by the fury of the tempest. The everlasting noise of the cataract,the deep roll of the thunder storm, or the sudden apparition of theeccentric comet in the heavens with its pale aspect and “ horrid hair,”