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the enlargement still continued to be equally perceptible. Prof. Powell, upon the grounds of such experiments, considers the conclusion to be unavoidable, that the enlargement is occasioned by some optical cause acting upon the formation of the focal image, and independent of any organic affection of the eye.

Pursuing this view of the origin of irradiation, he conceived that a striking verification of it would be obtained, if a photographic image of the object could be formed, exhibiting the enlargement. An experiment which he made with a view to decide this point, was attended with complete success. The image of the card when formed by a photographic process, either in the light of the bright sky or in the full sun, was found to exhibit a manifest enlargement.

Irradiation may be greatly diminished, or even completely destroyed, by the interposition of a lens between the object and the eye. Prof. Powell found that with ordinary daylight a lens which magnified three or four times was sufficient to extinguish every trace of the phenomenon. He states, as the result of his experience generally, that magnifying powers varying from ten to twenty effectually destroy the irradiation occasioned by the brightest light which the eye can bear.

M. Plateau experienced great difficulty in reconciling the effect produced by the interposition of a lens with the theory of irradiation, which ascribes its origin to some ocular cause. Prof. Powell is of opinion that it may be accounted for by the diffusion of light, resulting from the application of the lens, rendering the enlargement too faint to be perceptible. Upon the same principle he explains the absence of irradiation in experiments made with a double image micrometer. In this case, each of the images possesses only half the original brightness, and being at the same time viewed with a high magnifying power, the effect of irradiation might, by the combined operation of both these causes, be rendered insensible.

In observations with the telescope it has been found, that the regular enlargement occasioned by irradiation is mainly dependent on the aperture, focal length, and magnifying power of the instrument. With respect to the new moon, Prof. Powell remarks, that when viewed with a 30-inch achromatic, and very low powers, the projection of the illuminated crescent beyond the dark part of the disk was distinctly seen; with a power of 50 it was barely visible; and with 80 not at all. The following passage of a letter from Prof. C. Piazzi Smyth, cited by that philosopher, affords an interesting confirmation of the same fact:-" On a particularly fine night at the Cape, when the enlargement of the bright part of the new moon beyond the dark was unusually striking to the naked eye, amounting to two or three minutes of space, a 14-foot reflector was turned to the object, and, with every increase of power, the apparent projection was more and more cut down-definition was very good that night. With power of 300 the projection was barely sensible; it did, however, still absolutely exist, but to perhaps not more than two or three tenths of a second of space."

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It is generally admitted that the irradiation of bodies is affected by the dimensions of the aperture of the telescope. When the aperture is dimin ished, the apparent enlargement of the object is diminished also. This, doubtless, arises from the less intense brightness of the image. It is to be remarked, however, that when the aperture is reduced to very small

* Mém. Ast. Soc., vol. xviiii., p. 80.

dimensions, the diffraction of the object-glass begins to affect the apparent magnitude of the image in the opposite direction, so that when the aperture has reached a certain degree of contraction, the enlargement arising from this cause may compensate for the diminished effect of irradiation. Prof. Powell is of opinion that this may account for Dr. Robinson having found the interval between the transits of the sun's limbs to be unaltered by contracting the aperture of the telescope, and also for the fact that Mr. Dawes, during the transit of Mercury over the sun's disk in 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 suggest the conclusion, that the knowledge we possess respecting its nature and laws is still very imperfect and obscure. On the other hand, no doubt can exist that it exercises a sensible influence on the observations of the astronomer. It is manifest also, that as the art of observation continues to advance towards perfection, it will become more and more imperative on the part of the astronomer to attend to the effects produced by it. In the present state of the subject the inquirer is impeded at the very outset of his researches, by the want of data of sufficient completeness and precision upon which he might establish his reasoning. It is to be hoped that amid the activity which pervades every department of astronomical science, this branch of enquiry will not fail to receive an amount of attention adequate to its growing importance.

We shall now proceed to give a brief account of the researches of astronomers, 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 undergoes during an Eclipse.-Its explanation by M. Arago.-Corona of Light observed around the Moon.-Allusions made to it by Ancient Authors.-Explanations of its Physical Cause by different Individuals.-Protuberances on the Moon's Limb.-Their most probable nature.-Observations on the surface of the Moon during Eclipses— Undulations observed on the occasion of the Eclipse of 1842.-Similar Phenomena observed during the Eclipse of 1733.-Explanation of their Origin.-Optical Phenomena 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 from the operation of nature's laws, exciting alternately the admiration and delight of the attentive observer, there are some whose surpassing magnificence extorts universal homage, and awakens in the spectator a feeling of reverential awe. It is impossible for the most frivolous mind to regard with indifference the irresistible impetuosity of the hurricane as in its wild career it sweeps along every opposing obstacle, or to contemplate without emotion the sublime spectacle of the ocean when its billows are agitated by the fury of the tempest. The everlasting noise of the cataract, the deep roll of the thunder storm, or the sudden apparition of the eccentric comet in the heavens with its pale aspect and "horrid hair,"

seldom fails to arouse the apathy of the most listless votary of nature. But perhaps on no occasion does the display of stupendous power in the economy of the physical universe exercise so subduing an influence over the mind, or produce so humiliating a conviction of the impotence of all human efforts to control the immutable laws of nature, and arrest the course of events, as when the glorious orb of day, while riding in the hea vens with unclouded splendour, begins to melt away from an unseen cause, and soon totally disappears, leaving the whole visible world wrapped in the sable gloom of nocturnal darkness. The scene is rendered still more impressive by the circumstances accompanying so remarkable an occurrence. The heavens assume an unnatural aspect, which excites a feeling of horror in the spectator; a livid hue is diffused over all terrestrial objects; the plants close up their leaves as on the approach of night; the fowls betake themselves to their resting-places; the warbling of the grove is hushed in profound silence. Universal nature seems to have relaxed her energies, as if the pulse which stimulated her mighty movements had all at once stood still.

During the early history of mankind, a total eclipse of the sun was invariably regarded with a feeling of indescribable terror, as an indication of the anger of the offended deity, or the presage of some impending calamity; and various instances are recorded of the extraordinary effects produced by so unusual an event. In a more advanced state of society, when science had begun to diffuse her genial influence over the human mind, these vain apprehensions gave place to juster and more ennobling views of nature; and eclipses generally came to be looked upon as necessary consequences flowing from the uniform operation of fixed laws, and differing from the ordinary phenomena of nature, only in their less frequent occurrence. To the astronomer they have in all ages proved valuable in the highest degree, as tests of great delicacy for ascertaining the accuracy of his calculations relative to the place of the moon, and hence deducing a further improvement of the intricate theory of her movements. In modern times, when the physical constitution of the celestial bodies has attracted the attention of many eminent astronomers, observations of eclipses have disclosed several interesting facts, which have thrown considerable light on some important points of enquiry respecting the sun and moon.

In the present advanced state of astronomy, the records of ancient eclipses have been successfully employed in fixing the dates of contemporary events, and thereby rescuing history, in many instances, from the confusion usually incident to a remote antiquity. It is thus that phenomena, which, at the time of their apparition, were regarded as the mysterious heralds of some impending calamity in the moral world, totally independent of the ordinary course of nature, and which would have been allowed to pass unnoticed, had it not been for the terror which they inspired, have become subservient in illustrating many obscure passages in the writings of the persons who record them, and establishing our knowledge of ancient times upon a more satisfactory basis. This is justly regarded as one of the most remarkable triumphs of modern science.

Before entering upon the history of total eclipses of the sun it may not be out of place to give a brief description of the nature of eclipses in general, and of the various circumstances which determine their occur

rence.

The moon, in the course of her monthly revolution round the earth,

appears to describe a great circle in the heavens, inclined at an inconsiderable angle to the ecliptic. Hence it happens that, when the time of her conjunction with the sun coincides with her passage across the ecliptic, she appears projected upon the region of the heavens occupied by the sun, and, being an opaque body, conceals the whole or a portion of his disk from a spectator on the earth. The defect of light arising from this cause is what is termed a solar eclipse. It is manifest that the moon cannot wholly intercept the solar light, unless during conjunction her apparent diameter exceed the apparent diameter of the sun. This condition, however, is not always fulfilled, for, as the moon revolves in an orbit of considerable eccentricity, her distance from the earth varies in a sensible degree, and hence arises a corresponding variation in her apparent diameter, causing it sometimes to exceed the apparent diameter of the sun, and other times to fall short of it. Hence, when the centres of the sun and moon are projected upon the same point in the heavens, and the apparent diameter of the moon at the same time exceeds that of the sun, the solar disk will be entirely concealed from the view of the spectator, and a total eclipse will be the consequence. If, however, while all other circumstances are alike, the apparent diameter of the moon be less than that of the sun, the solar disk will not wholly cease to be visible, but the opaque body of the moon will appear projected centrally on it, leaving a narrow ring of light exposed to view. In this case, there occurs the interesting phenomenon of an annular eclipse. When the moon does not pass centrally over the sun, but merely overlaps a segment of his disk, a portion of his light only is prevented from reaching the place where the spectator is situate, and a partial eclipse then ensues.

During a total eclipse of the sun it is manifest that the place on the surface of the earth which happens from this cause to be obscured, is then situated within the moon's shadow. When only a portion of the solar light is intercepted by the moon as in the case of an annular or partial eclipse, the place to which the eclipse is visible is situate beyond the limits of the lunar shadow in a partially-obscured region termed the penumbra.

Since the moon in passing between the sun and the earth intercepts the solar light, either wholly or partially from some place on the surface of the latter body, so when, in the course of her synodic revolution, she arrives in the opposite point of her orbit, the earth being now interposed between her and the sun, she is deprived in her turn of the solar light, and the phenomenon of a lunar eclipse is the consequence. It is to be remarked that in all lunar eclipses, the solar rays are supposed to be wholly intercepted from the obscured surface, those cases in which a partial deprivation of light only occurs, being left out of consideration, on account of the difficulty of determining the exact instant when the obscuration begins or ends. Hence a lunar eclipse corresponds to a total eclipse of the sun in regard to its specifie character, the surface obscured being, in each case, actually immersed in the shadow of the eclipsing body.

If the moon revolved in the plane of the ecliptic, she would necessarily pass between the sun and the earth at every conjunction, while again at every opposition, the earth would be interposed between the sun and moon. Hence a solar eclipse would take place at every new moon, and a lunar eclipse at every full moon. The lunar orbit being, however, inclined at an angle of about 5° to the plane of the ecliptic, an eclipse, whether solar or lunar, can only occur, when the moon during conjunction or opposition is near either of the nodes of her orbit. Astronomers have found by calcula

tion that a solar eclipse cannot take place unless the moon during conjunction be within 17° 21′ 27′′ of her node; nor a lunar eclipse unless during opposition her distance from the node be less than 11° 25′ 40′′. The question with respect to the specific nature of the eclipse which may happen in either case, whether it be total or partial, will depend, cæteris paribus, upon the distance of the moon within the assigned limit.

It follows from the foregoing remarks that, whenever the lunar node returns to the same position with respect to the sun and moon, an eclipse of the same nature will always recur. It is manifest that the frequent occurrence of eclipses in general, will depend on the condition of the lunar nodes with respect to their being fixed or moveable. Now, it appears from observation, that they regress with a rapid motion on the ecliptic, making a complete tour of the heavens in about 18 years. A remarkable relation subsists between the synodic revolution of the moon, and the motion of her nodes, which causes the phenomena of eclipses to return within a definite period nearly in the same order. It appears, in fact, that while 223 lunations include 6585.321 days, the nodes return to the same position with respect to the sun in 6585.772 days. The difference amounts to .451 of a day, or barely 12 hours, during which interval of time the sun describes an arc of 28′ 6′′ relative to the lunar node. Hence at the end of 6585.321 days the moon will have returned to the same position with respect to the sun, and will only be at a distance of 28' 6" from the same position with respect to the node. This period of 223 lunations or 18 10d 7h 43m*, which occasions a recurrence of eclipses in the same order, was known to the Chaldeans, who arrived at its discovery by comparing together the records of eclipses extending throughout a long succession of ages t. It appears from theory that seven eclipses may and that two must take place in the course of every year. When the number of eclipses is the greatest possible, both eclipses are solar. Out of seventy eclipses which usually take place within a period of 18 years, the average number of solar eclipses is forty-one, and of lunar twenty-nine.

Although solar eclipses, generally speaking, occur very frequently, a total eclipse of the sun is an event which happens only on very rare occasions. This arises from the circumstance of the moon being so small a body compared with the sun, that her shadow frequently does not extend so far as the earth, and even when it does, its dimensions are so inconsiderable that only under conditions of the most favourable nature, can any portion of the earth pass through it. In fact, although an eclipse recurs nearly about the same time within a period of 223 lunations, it will not be exactly of the same magnitude, and the alteration, although small, may suffice to transform it from a total to a partial eclipse.

If a total eclipse of the sun is an event of rare occurrence even anywhere on the surface of the earth, still more especially is this true when

It will be 18 10 7 43, or 18 11d 7h 43m, according as four or five leap years happen within the cycle.

In these calculations it is only the mean synodic revolution of the moon, and the mean motion of her node, that are considered. It is clear, therefore, that even although the two periods alluded to in the text were precisely equal, the inequalities both of the moon's longitude and of her node would cause the corresponding eclipses in each cycle to be of somewhat different magnitudes, and also to recur with more or less irregularity. The cycle is sufficiently true, however, to enable a person by means of it to predict eclipses in a rough way, and this is all that the Chaldean astronomers aspired to accomplish.

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