HISTORY OF PHYSICAL ASTRONOMY.

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to the outer border, the yellow gradually became fainter, until at length itseemed almost quite white *.

Ferrer, in his account of the total eclipse of 1806, has given a briefdescription of the luminous ring. He states, that the colour of the lightemitted by it resembled pearl colour, and that it had a breadth of about O'.From the exterior margin there were seen luminous rays extending out-wards to a distance of more than 3°. The light was brightest at the edgeof the moon, and terminated very confusedly at the outer border. Withrespect to its relative position, the ring seemed to be concentric with thesunf. De Witt, who observed the same eclipse at Albany in the State ofNew York, states, that “ the luminous circle on the edge of the moon,as well as the rays which were darted from her, were remarkably pale, andhad that bluish tint which distinguishes the colour of quicksilver from adead white.” J

The luminous ring appeared with great splendour during the totaleclipse of July 8, 1842. It was of uniform brightness in those partsthat bordered on the moon’s limb, whence it faded imperceptibly out-wards, terminating so confusedly, that it was impossible to trace its ex-terior limit. The difference between the inner and outer parts of thering appeared to M. Arago to be sufficiently marked to sanction thesubdivision of the ring into two concentric zones, the inner zone beingeverywhere of equal density and well defined at the outer border, while onthe other hand the exterior zone, although the broader of the two, wasfainter even at the inner border, and gradually diminished in brightnessuntil it was lost in the surrounding darkness. The interior zone was aconspicuous object at all the stations where the eclipse was observed, andeverywhere presented the same aspect. The exterior zone, being muchfainter, did not exhibit the same degree of magnitude at the different placesof observation. Indeed it was only under favourable conditions of theatmosphere that it was visible at all.

With respect to the dimensions of the corona, it could not be expectedthat the observations would present a very close agreement, owing to theindefinite nature of its structure. At Perpignan , M. Silva found by meansof a repeating circle, that the interior zone had an invariable breadth of 3'during the whole time of complete obscuration. Mr. Airy estimated itsbreadth at an eighth part of the lunar diameter, or about 4'. From thestatements of the various observers, it would appear that this part of theluminous ring, or in other words the part exhibiting a uniform condensation,extended from the moon’s limb to a distance of between 3' and 4'.

The exterior zone of the corona being more or less perceptible accord-ing to the condition of the atmosphere, there naturally arose considerablediscordances in the observations relative to its breadth at the differentstations. At Montpellier , M. Petit obtained 8' 45" for the distance towhich the corona was visible. This would indicate the breadth of the ex-terior part to be about 5'. Mr. Baily estimated the breadth of the wholecorona at half the moon’s diameter, or about 16 / . At Lipesk, where itappeared with great splendour, M. Otto Struve found that it was visibleto a distance of 25' from the moon’s limb.

At several stations the regularity of the contour of the ring appearedto be interrupted by two or more expansions of light. In France there

* Phil. Trans., 1779, p. 108.

t Trans. Amer. Phil. Soc., vol. vi., p. 266. f Ibid., p. 801.