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HISTORY OF PHYSICAL ASTRONOMY.

nebulaB, which previous astronomers had represented as possessing around uniform disk, were found by Lord Rosse to exhibit unequivocalindications of an annular structure. Nebulae of this description areextremely rare in the heavens. Only four of such objects had hithertobeen recognised by astronomers, two in the northern and two in thesouthern hemisphere .

The class of objects comprehended under the designation of NebulousStars, whose appearance Sir William Herschel was unable to account for,except by assuming the existence of a shining fluid existing independ-ently in space, have also exhibited, in the telescopes of the distinguishednobleman just referred to, au aspect which entirely obviates the necessityof having recourse to such an hypothesis. One of the most interestingobjects of this class is the nebulous star marked No. 4-50 in Sir JohnHerscliel’s catalogue of 1833. The star-like point of this object, whenviewed in the 6-feet reflector, is seen to be placed in the centre of anebulous nucleus, and beyond there appears a nebulous ring completelyseparated from the latter. Another nebulous star, i Orionis, has beenfound to exhibit a somewhat analogous aspect. The pale nebulous lightsurrounding the star is found to contain a dark cavity, not exactly sym-metrical with respect to the star.

Of the other nebulas examined by Lord Rosse , one of the most inte-resting is the one marked No. 2098 in Sir John Herschel’s northerncatalogue. This object has been found to exhibit ansa, indicating, inall probability! a nebulous ring seen edgeways. This phenomenon appearsto afford a valuable experimentum crucis in deciding the question relativeto the structure of those nebulae which exhibit an annular aspect. Itis not difficult to see that such an appearance might be produced eitherby a real ring or by a hollow spherical shell of nebulous light. In thelatter case, however, it would always exhibit a perfectly round appear-ance ; whereas, in the former, there is only one position of the ringin which it would appear round. In every other position it would appearmore or less elliptical, and when its plane passed through the earth, itwould resemble two opposite ansa attached to the central object, providedthere was one. Now r , since observation has, in one instance at least, dis-closed indications of such ansa, and thereby afforded unequivocal evi-dence of the actual existence of a ring composed either of stars or ofa nebulous substance, we may reasonably conclude that the annularappearance exhibited by several other nebute is occasioned by a similarcause, and not by a hollow sphere of nebulous light. We are alsowarranted by the same consideration in supposing that those ellipticalnebulas which exhibit dark chinks are in reality nebulous rings of moreor less regular structure, seen obliquely with respect to the line of vision,and not hollow elliptical shells of nebulous light.

Sir John Herschel has remarked, in the paper containing his catalogueof nebulae visible in the northern hemisphere, that further discoveries ofsuch objects can only be expected to result from the use of telescopes pos-sessing a higher degree of optical power than a reflector of eighteeninches’ aperture, the instrument with which all the nebulae hitherto knownto exist were mainly discovered. The observations of Lord Rosse areplainly destined to be instrumental in realising this expectation. Althoughno direct search for new nebula has yet been undertaken by his lord-ship), several objects of this class, which it was impossible to discern ininstruments of inferior power, have been incidentally revealed by the im-mense light of the 6-feet reflector. It is thus evident that, with a greater