HISTORY OF PHYSICAL ASTRONOMY.
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first sight this explanation appears very plausible, but it is to be remarkedthat the principle of an ethereal fluid pervading the celestial regions, uponwhich it is founded, is unsupported by any positive evidence of a trust-worthy character, and therefore it cannot be viewed in any other light thanas a gratuitous assumption.
Other persons have sought an explanation of the luminous ring in thezodiacal light. It is to be remarked respecting this appendage of the sun,that it does not encompass his disk symmetrically, being somewhat elon-gated in the direction of the ecliptic. Dominic Cassini, to whom its dis-covery is due, remarked, that if it were palpably visible, it would probablyappear as a faint chevelure about the sun*. Some slight traces of an elonga-tion of the luminous ring which appears around the moon during a totaleclipse of the sun, have led to the surmise that it may possibly be noother than the zodiacal light. In the vast majority of cases, however,the ring has been found to be quite circular; and even in those caseswherein luminous expansions have been observed at opposite extremitiesof a diameter of the ring, the elongation resulting therefrom does not takeplace in the direction of the ecliptic.
The most probable explanation of the luminous ring is that accordingto which it arises from the presence of an atmosjohere about the sun. Itsround figure, its nebulous structure, and its gradually-diminishing densityoutwards, are all favourable to the supposition of its being due to an elasticfluid encompassing the solar orb, and gravitating everywhere towards itscentre. It is true that precisely similar results would ensue from theexistence of an atmosphere about the moon; but, in fact, there is no reasonto suppose that the moon possesses an atmosphere capable of producingan appreciable effect. On the other hand, the hypothesis of a solar atmo-sphere is not only warranted by the analogy of the other bodies of theplanetary system, but is also supported by evidence of a positive naturederived from observations on the physical constitution of the sun. Thechanges presented by that body when viewed in the telescope can only beconsistently accounted for by the supposition of two dissimilar envelopesof matter suspended in a transparent atmosphere at different altitudesabove his surface. That such a circumambient fluid really exists isshewn most unequivocally by the gradual diminution of the brightness ofthe solar disk tow-ards the margin. With respect to this fact, Mr. Airystates it is so palpable to observation, that when in the course of his ex-periments he had occasion to throw a portion of the sun’s disk upon a smallscreen, he was always able to determine whether the limb was approachingthe edge of the fully-illuminated screen simply by the change of the inten-
pears to contain the germ of the discovery of the diffraction of light usually ascribedto Grimaldi. Having inserted two small apertures in the window of a dark chamber, oneof which was about the size of a grain of millet, and the other equal in magnitude to a pea,he drew two circles upon a screen placed opposite, the diameters of which bore the sameproportion to each other as those of the apertures. The light of the sun being then ad-mitted through the larger aperture, depicted an image on the screen which exactly coin-cided with the larger circle; but when this aperture was closed, and the light was admittedthrough the smaller one, the image was no longer well defined, its margin was somewhatdusky, and finally it surpassed in magnitude the smaller circle.
* Cassini first began to see the zodiacal light in March, 1683; and in the Journal desSavans for June of the same year, he made the remark alluded to in the text. For anaccount of the observations (extending over several years) by means of which he defi-nitely established the existence of this phenomenon, See Anc. Mem. Acad, des Sciences,tom. viii., p. 121, et seq.