The class of spiral nebulæ included, in 1850, fourteen members, besides several in which the characteristic arrangement seemed partial or dubious.[331] A tendency in the exterior stars of other clusters to gather into curved branches (as in our Galaxy) was likewise noted; and the existence of unsuspected analogies was proclaimed by the significant combination in the "Owl" nebula (a large planetary in Ursa Major)[332] of the twisted forms of a spiral with the perforated effect distinctive of an annular nebula.
Once more, by the achievements of the Parsonstown reflector, the supposition of a "shining fluid" filling vast regions of space was brought into (as it has since proved) undeserved discredit. Although Lord Rosse himself rejected the inference, that because many nebulæ had been resolved, all were resolvable, very few imitated his truly scientific caution; and the results of Bond's investigations[333] with the Harvard College refractor quickened and strengthened the current of prevalent opinion. It is now certain that the evidence furnished on both sides of the Atlantic as to the stellar composition of some conspicuous objects of this class (notably the Orion and "Dumb-bell" nebulæ) was delusive; but the spectroscope alone was capable of meeting it with a categorical denial. Meanwhile there seemed good ground for the persuasion, which now, for the last time, gained the upper hand, that nebulæ are, without exception, true "island-universes," or assemblages of distant suns.
Lord Rosse's telescope possesses a nominal power of 6,000--that is, it shows the moon as if viewed with the naked eye at a distance of forty miles. But this seeming advantage is neutralised by the weakening of the available light through excessive diffusion, as well as by the troubles of the surging sea of air through which the observation must necessarily be made. Professor Newcomb, in fact, doubts whether with _any_ telescope our satellite has ever been seen to such advantage as it would be if brought within 500 miles of the unarmed eye.[334]
The French opticians' rule of doubling the number of millimetres contained in the aperture of an instrument to find the highest magnifying power usually applicable to it, would give 3,600 as the maximum for the leviathan of Birr Castle; but in a climate like that of Ireland the occasions must be rare when even that limit can be reached. Indeed, the experience acquired by its use plainly shows that atmospheric rather than mechanical difficulties impede a still further increase of telescopic power. Its construction may accordingly be said to mark the _ne plus ultra_ of effort in one direction, and the beginning of its conversion towards another. It became thenceforward more and more obvious that the conditions of observation must be ameliorated before any added efficacy could be given to it. The full effect of an uncertain climate in nullifying optical improvements was recognised, and the attention of astronomers began to be turned towards the advantages offered by more tranquil and more translucent skies.