388

HISTORY OF PHYSICAL ASTRONOMY.

lity with the disk. When the area of the rays was not concentric withthe disk, the luminous ring appeared broadest at the part where the raysapproached nearest the edge of the disk.

In order to establish the experiment more fully, the ring was examinedin a telescope with a magnifying power of 20, furnished with cross wiresin its focus. Before the light was allowed to fall upon the opaque disk,the edge of the latter was made to coincide with the intersection of thewires; but as soon as the light was admitted through the aperture, thering was seen extending sensibly beyond. The intersection of the wireswas now made to coincide with the outer edge of the ring, and upon theexclusion of the light the disk was seen to have fallen sensibly within.

When the aperture of the telescope was diminished to ^ of an inch, thering still continued to be distinctly seen, but the diffraction produced bythe contraction of the aperture now caused the fringes to be also visible.

According to Prof. Powell, the colour of the ring was yellowish. Therealso appeared a faint bluish light extending within the dark circular areaencompassed by it.

The circumstances of the above-mentioned experiment, abstractedly con-sidered, bear a considerable resemblance to those which determine the ap-pearance of the luminous ring around the moon during a total eclipse ofthe sun. Moreover, the results may be said also to agree in their mainfeatures. It would seem, therefore, not improbable, that they are attri-butable to the same physical cause. This conclusion, however, has beenobjected to, on the ground of the resemblance between the natural andartificial eclipses not being sufficiently perfect, since, in the case of the arti-ficial eclipse, the diffracting body is surrounded by a medium possessing aconsiderable power of reflecting the solar light. Again, it has been re-marked by Sir David Brewster , that in all experiments similar to thoseabove alluded to, the breadth of the ring is totally independent of themagnitude of the diffracting body, and therefore the unavoidable con-clusion is, that in the case of the natural eclipse, the ring would beutterly invisible on account of the comparatively immense distance ofthe moon from the earth *. This must be considered as a fatal objectionto the above explanation, if the principle upon which it is founded beadmitted to he true. But besides, the diffraction theory is incapable ofoffering any account whatever of many of the subordinate features ofthe ring, and therefore upon this ground alone, it cannot be consideredas affording a faithful representation of the phenomenon.

While some persons have sought to explain the luminous ring, by as-suming it to be an appendage of the moon, others have endeavoured toeffect the same object by referring its origin to the sun. It has beenalready mentioned that the illustrious Kepler , who had the merit of firstdirecting the attention of astronomers to the ring, suggested that it mightarise from the refraction of the lunar atmosphere. Another hypothesisadvanced by him, as probably involving the true explanation of the pheno-menon was to the effect that it was occasioned by the combustion of theether in immediate contact with the sun j-. It cannot be denied that, at

* Brewster ’s Edin. Encyclop., Art. “Astronomy.”

+ Ad Vitellionem Paralipomena, p. 301; Epitome Astronomiae, p. 893. In orderto account for the circumstance of the darkness not being equally intense during all totaleclipses of the sun, Kepler imagined the ether to be sometimes very dense, and thereforeeasily inflamed; while, on the contrary, at other times being very rare and limpid, it wasless liable to be affected by the action of the solar rays. In his explanation of this hypo-thesis, he mentions an experiment which it may not be uninteresting to describe, as it ap-