The occultations of Aldebaran approaching again in the years 1829 and 1830, the Society just mentioned invited the particular attention of astronomers to thew. The consequence a large number of communications from different parts of Europe, which are printed in the fourth volume of their Memoirs. Nothing can be more different than the results : some, now saw the phenomenon for the first time; others, who Lad seen it before, did not ace it ; some, who had never seen it before, continued unable to do mo. Of six observers at the Royal Observatory, five distinctly saw the projection on the moon's limb, and one maw it hang on the edge of the moon five or six seconds before it disappeared. Of three at the Observatory of Paris, two distinctly saw the projection, and one mw the star disappear inetan taneotraly, leaving a shade onabre") on the hart of the moon at which it dbappeared. The majority saw the star either projected or hanging on the moon's edge. It is to be noted that this phenomenon has been seen at the dark edge of the moon as well as at the enlightened. Its cause is matter of much diversity of opinion.
The meet recent explanation Is due to Mr. Airy, who in the year 1$59 communicated a per on the subject to the Royal Astronomical Society, which is to be found in the twenty-eighth volume of the ' Memoirs' published by that body. Mr. Airy remarks, " that on the principles of the undulatory theory of light, the image in a telescope of any lumlnone point, whether a star or a portion of the illuminated surface, is not a point." Thin is exemplified in the case of the bright rings which surround the image of a star as seen in a good teleseope. It is plain also, for the same roman, that the aggregate of light produced by the aggregate of all the Intuitions points of the moon's disc, lo not a luminous imago bounded by a sharp outline, at what we consider the geometrical outline of the image, but that the geometrical outline is fringed by a band of illumination, produced by the interlacing and superposition of all the systems of rings. Hence it
follows that when, with a very fine telescope we see the moon's limb very sharply defined, and apparently surrounded by immediate Harkness, we do in reality ace it erroneously. 31r. Airy conceives that during ordinary observations of the moon, the conviction that the outline of light ought to correspond to a given curve, may affect the visual organs so as to incapacitate them from hwreciving the fainter light beyond that curve ; but that in the excitement and intentness of observing an occultation, the state of the sensational organisation is probably much changed, and as the presumed time of the phenomenon comes nearer and nearer, the eye probably becomes more and more sensible to the faint diffused light, and the visible boundary of light extends further and further into the darkness. In numerous instances when the boundary of the moose light has swelled till it touches the star, it swells no further, and the star hangs on the moon's limb. According to 31r. Airy, it seems perfectly conceivable, that the mental coutemplation of the relation of the positions of the moon and star which is implied in the phenomenon that is to be observed, may frequently so far act on the sensibilities, that when that relation (namely, contact) is once gained, the mental effort does not make the sense more acute than is necessary, and may even somewhat relax as the denser light of the moon reaches the star. This explanation, it will be seen, refers the phenomenon to the principle of the irradiation of light.