HISTORY OP PHYSICAL ASTRONOMY.
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cultation* . No satisfactory explanation of this strange anomaly has hithertobeen advanced by any person. One of the most recent, as well as one ofthe most specious modes of accounting for its physical origin is due toProf. Stevelly. According to the hypothesis proposed by him, the phe-nomenon is purely an effect of the diffraction of light. Newton hadshewn by experiment that the rays of light which pass very near theedge of a body are bent away from it, so as for a short distance to describecurves which are convex with respect to it. Prof. Stevelly conceives thevisible contour of the moon to be such a diffracting edge to the slenderbeam of light which reaches the eye from a fixed star. Under such cir-cumstances, it is manifest, from the convexity of the course pursued bythe beam of light while under the influence of the diffracting force, thatits final direction when it enters the eye ought, if produced toward themoon’s surface, to fall within her limbf.
If the above explanation were true, the phenomenon ought to be visibleon the occasion of every occultation of a star by the moon. Such, how-ever, is far from being the case. This is a defect which may be said tocharacterise in a greater or less degree all the explanations that havehitherto been offered relative to a phenomenon, which is manifestlyan optical illusion, although its true source is not easy to be discovered.
Sometimes the planets occult one another in the course of their motionround the sun. Such phenomena, however, are manifestly of very rareoccurrence. Kepler states that on the 9th of January, 1591, Msestlinand himself witnessed an occultation of Jupiter by Mars . The red colourof the latter on that occasion plainly indicated that it was the inferiorplanet |. He also mentions that on the 3rd of October, 1590, Meestlinwitnessed an occultation of Mars by Venus . In this case, on the otherhand, the white colour of Venus afforded a clear proof that she was thenearer of the two planets to the earth §. It is to be borne in mind thatthese observations were made before the invention of the telescope, so thatit is doubtful whether in either of these cases the one planet was actuallysuperposed above the other, for the peculiar colour of the light mightarise simply from the predominating influence of the brighter planet ||.
Sometimes there happen occupations of the fixed stars by the planets.On the 1st of October, 1672, the planet Mars eclipsed a star in the con-stellation Aquarius. Cassini, who was then engaged in researches on theparallax of Mars , had previously resolved to observe this interesting phe-nomenon, but he was prevented by cloudy weather from effecting his ob-ject. Phenomena of a similar nature have occurred in more recent times,but no physical consequences have been deduced from them.
* See a collection of observations of this nature in a paper by Sir James South onthe occultation of S Piscium by the moon (Mem. Ast, Soc., vol. iii., p. 303, et seq).
f Brit. Assoc. Rep., 1845. (Trans, of the Sections, p. 5.)
J Ad Vitellionem Paralipomena, p. 305. § Ibid., p. 305.
II For an account, by Dr. Bevis, of an occultation of Mercury by Venus , on May 17,1737, see the Philosophical Transactions for 1737, p. 394. On this occasion, the inte-resting phenomenon alluded to by Kepler does not seem to have been remarked by anyobserver.
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