for the purpose of ascertaining whether any effect would be produced on the irradiation. When the disk was in its state of greatest illumination no change was perceptible; but on turning off the light from the field of view, when the disk was obscured by the interposition of the oiled paper, a manifest enlargement was immediately apparent. The absence of any sensible enlargement in the first instance, arose doubtless from the circumstance of the brilliant light of the disk effectually overpowering the fainter light diffused over the field of view.

As it appeared evident from these experiments that the irradiation of an object was sensibly influenced by the brightness of the ground on which the object was projected, Dr. Robinson imagined that its effect might be eliminated altogether from observations of the sun and moon by sufficiently illuminating the field of view of the telescope. To effect such an illumination, however, by an artificial process was absolutely impossible, when the question related to observations of the sun, nor was it even generally practicable in the case of the moon. Under these circumstances, Dr. Robinson hit upon the ingenious idea of illuminating the field of view by deriving the light from the luminary itself. In pursuance of this design he proposed to cover the object-glass of the telescope with a semi-transparent diaphragm, leaving a small central aperture to admit the rays which formed the image at the focus, while the rest of the diaphragm served to fill the field with scattered light. This method was applied by Dr. Robinson to observations of the sun, and the results obtained by him in a few instances, appear to have been very satisfactory; the value of the solar diameter determined by means of the transit instrument, exceeding only by a very small quantity, the value assigned by the tables.

One curious fact noticed by Dr. Robinson, in the course of his experiments, is worthy of mention. Having examined the luminous disk with a double image micrometer, he was surprised to find that the contact of the two images was not affected by the intensity of their illumination. He expressed himself as somewhat doubtful of the result, the experiment having been hastily made. It would appear, however, to be supported by experiments of a similar nature made by other astronomers. M. Arago had previously attempted to ascertain the influence of irradiation by measuring with a rock crystal micrometer the apparent diameters of luminous disks, and then comparing them with the corresponding results obtained by combining the absolute diameters with the distance of the disks from the eye; but in all such experiments he found that the irradiation was insensible, even when the illumination of the disks was more intense than that of the full moon *. A similar result was obtained by M. Bessel when he sought by means of his great heliometer to deter mine the influence of irradiation on the occasion of the transit of Mercury across the sun's disk in the year 1832. Since, in this case, the effect of irradiation is to dilate the apparent diameter of the sun, and diminish that of the planet, it is not difficult to see that if such an effect really existed in any sensible degree, the luminous thread which succeeds the first interior contact of the two bodies ought to acquire instantaneously a certain degree of breadth at the point of contact; and similarly the luminous thread which precedes the second interior contact ought to exhibit a sudden rupture at the point of contact as if some protuberance had all at once been formed on the planet. Bessel was unable to discern the slightest

• Traité d'Astronomie Physique, par M. Biot; 2ieme edit., tome ii., P. 534.

indication of either of these phenomena during the transit of the planet above referred to. But he obtained a still more unequivocal proof that the effect of irradiation was insensible. Since the first interior contact of the two bodies is indicated by the sudden closing together of the two extremities of the luminous border of the sun, while the second interior contact is made apparent by the equally sudden rupture of the opposite border, it is manifest that the interval of time included between the two contacts when taken in conjunction with the relative motion of the planet, as assigned by the tables, will serve to determine the apparent diameter of the sun. Now Bessel, by employing this method, arrived at a result which coincided exactly with that obtained by direct measurement of the solar diameter with a micrometer during the transit of the planet.

Thus it appears that the experiments of Bessel concur with those of M. Arago and Dr. Robinson in indicating that the effect of irradiation is insensible when the instrument employed in measuring it is one which gives a double image of the luminous body. We shall presently have occasion to mention that experiments of this nature have been supposed to furnish only particular illustrations of a general law affecting the irradiation of two luminous objects placed in juxtaposition.

In the year 1838 M. Plateau communicated to the Royal Academy of Brussels, an elaborate memoir, containing an account of a variety of experiments performed by him with the view of elucidating the nature and laws of irradiation *. A very simple mode of exhibiting the enlargement arising from this cause was devised by him. A white card was divided into six rectangular compartments, by drawing two parallel lines pretty close to each other down the middle of the card, and then bisecting these by a third line, drawn at right angles to them. The middle rectangle in the upper half of the card, and the broad lateral rectangles in the lower half, were then painted black, so that there appeared in the middle of the card a band composed of two parts, the one black, (in the upper half of the card.) projected upon a white ground, the other white, (in the lower half,) projected upon a black ground. This contrivance manifestly tended to render the effect of irradiation more perceptible; for while the white band in the lower half of the card was liable to dilate from the encroachment of the light upon the black ground, the black band in the upper half was equally liable to contract from the encroaching irradiation of the white light on each side of it. The card being placed vertically near a window, so as to be well exposed to the light, was then viewed at the distance of a few yards, when the effect of irradiation was clearly exhibited by the white band in the middle of the lower half appearing sensibly broader than the black band above it. The following are the more important conclusions to which M. Plateau was conducted by his experimental researches on the subject of irradiation.

1° The quantity of irradiation increases with the brightness of the object, but in a much less rapid proportion. It has very nearly attained its maximum when the brightness is equal to that exhibited by the northern region of the sky.

2° Two irradiations in close proximity tend to neutralize each other. The diminution arising from this cause is greater in each case, as the interval between the luminous objects is less.

3° The quantity of irradiation augments with the time of contemplating

• Nouv. Mém. de l'Acad. Royal de Bruxelles, tome xi., p. 1, et seq.

the object. It is different for different individuals, and for the same individual it varies from one day to another.

4° When an object is viewed through a telescope, the apparent enlargement exhibited by it arises from two distinct causes, viz., ocular irradiation, and the aberration of the telescope. The part due to ocular irradiation depends on the magnifying power of the telescope, on the brightness of the image, and on the physiological qualities of the eye of the observer. Moreover, the interposition of the eye-glass of the telescope tends to produce a peculiar effect on the enlargement.

5° The part of the enlargement due to the aberration of the telescope varies necessarily with the instrument employed. For the same instrument, it may be considered as constant.

The most remarkable of these propositions is that wherein it is announced that the irradiations of two luminous objects are diminished by their mutual proximity. M. Plateau extends this principle to the phenomena of irradiation seen with the naked eye as well as to those observed in the telescope. He rests its demonstration upon various experiments, some of which are of a very convincing nature, and do not seem to be liable to any objection. He refers to this principle the absence of any sensible irradiation in the experiments of Arago, Robinson, and Bessel, above alluded to. Without expressing a formal opinion on its origin he seems disposed to consider it as due to the fact, that there does not exist in either case a sufficient contrast between the luminous object and the ground upon which it appears projected. M. Plateau concurs with most other philosophers in placing the source of irradiation in the eye, but he does not express any opinion respecting the mode by which the effect is produced.

The most recent account of experimental researches on Irradiation, is contained in a paper communicated by Professor Powell to the Astronomical Society, in the year 1849. That distinguished philosopher appears disposed to regard the phenomena of irradiation as due, in a great degree, to some cause extraneous to the eye. While admitting that the irregular scintillation of a star may arise from some physiological affection of the organ of vision, and may vary in different individuals, he considers that the enlargement exhibited by a well-defined disk, capable of exact measurement, cannot reasonably be ascribed to a cause of so fluctuating a nature. He then proceeds to describe various experiments which tend to support this assertion. One of the most unequivocal results derived from these experiments consisted in this, that an image of the object, formed in the focus of a lens, is affected by irradiation in an equal degree with the object itself. A card, similar to that employed by M. Plateau, being exposed to a moderately strong light, and its image being then thrown by reflexion upon a ground glass placed in the focus of a lens, the image of the card was seen painted on the ground glass, with precisely the same enlargement as that which it exhibited to the naked eye. It might be urged against this experiment, that the brightness of the light at the focus of the lens might in its turn produce its irradiation, so as to occasion the apparent enlargement of the image. With reference to this objection Prof. Powell remarks, that the image on the ground glass was in general by far too faint to produce any such secondary effect. That the irradiation could not be due to this cause was proved further beyond all doubt by the fact, that when a dark glass was interposed between the eye and the image of the card in the focus,

* Mém. Ast. Soc., vol. xviii., p. 69, et seq.

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.'

It is generally admitted that the irradiation of bodies is affected by the dimensions of the aperture of the telescope. When the aperture is diminished, 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,"