With greater telescopic power additional features begin to reveal themselves; the mist immediately above the trapezium assumes a roughly triangular shape, and is evidently much denser than the rest of the nebula, presenting a curdled appearance similar to that of the stretches of small cloud in a 'mackerel' sky; while from the upper jaw of the fish-mouth a great shadowy horn rises and stretches upward, until it gradually loses itself in the darkness of the background. This wonderful nebula appears to have been discovered in 1618, but was first really described and sketched by Huygens in 1656, since when it has been kept under the closest scrutiny, innumerable drawings of it having been made and compared from time to time with the view of detecting any traces of change. The finest drawings extant are those of Sir John Herschel and Mr. Lassell, and the elaborate one made with the help of the Rosse 6-foot mirror.
Drawing, however, at no time a satisfactory method of representing the shadowy and elusive forms of nebulæ, has now been entirely superseded by the work of the sensitive plate. Common, Roberts, Pickering, and others have succeeded admirably in photographing the Great Nebula with exposures ranging from half an hour up to six hours. The extension of nebulous matter revealed by these photographs is enormous (Plate [XXX.]), so much so that many of the central features of the nebula with which the eye is familiar are quite masked and overpowered in the photographic print. The spectrum of the Orion Nebula exhibits indications of the presence of hydrogen and helium, as well as the characteristic green ray which marks the unknown substance named 'nebulium.'
The appearance of this 'tumultuous cloud, instinct with fire and nitre,' is always amazing. Sir Robert Ball considers it one of the three most remarkable objects visible in the northern heavens, the other two being Saturn and the Great Cluster in Hercules. But, beautiful and wonderful as both of these may be, the Orion Nebula conveys to the mind a sense of mystery which the others, in spite of their extraordinary features, never suggest. Absolutely staggering is the thought of the stupendous dimensions of the nebula. Professor Pickering considers its parallax to be so small as to indicate a distance of not less than 1,000 years light journey from our earth! It is almost impossible to realize the meaning of such a statement. When we look at this shining mist, we are seeing it, not as it is now, but as it was more than a hundred years before the Norman Conquest; were it blotted out of existence now, it would still shine to us and our descendants for another ten centuries in virtue of the rays of light which are already speeding across the vast gulf that separates our world from its curdled clouds of fire-mist, and the astronomers of A.D. 2906 might still be speculating on the nature and destiny of a thing which for ages had been non-existent! That an object should be visible at all at such a distance demands dimensions which are really incomprehensible; but the Orion Nebula is not only visible, it is conspicuous!
Photograph of the Orion Nebula (W. H. Pickering).
The rival of this famous nebula in point of visibility is the well-known spiral in the girdle of Andromeda. On a clear night it can easily be seen with the naked eye near the star Nu Andromedæ, and may readily be, as it has often been, mistaken for a comet. Its discovery must, therefore, have been practically coincident with the beginnings of human observation of the heavens; but special mention of it does not occur before the tenth century of our era. A small telescope will show it fairly well, but it must be admitted that the first view is apt to produce a feeling of disappointment. The observer need not look for anything like the whirling streams of light which are revealed on modern long exposure photographs (Plate [XXXI.], 1). He will see what Simon Marius so aptly described under the simile of 'the light seen from a great distance through half-transparent horn plates'—a lens-shaped misty light, brightening very rapidly towards a nucleus which seems always on the point of coming to definition but is never defined, and again fading away without traceable boundary into obscurity on every side. The first step towards an explanation of the structure of this curious object was made by Bond in the middle of last century. With the 15-inch refractor of the Cambridge (U.S.A.) Observatory, he detected two dark rifts running lengthwise through the bright matter of the nebula; but it was not till 1887 and 1888 that its true form was revealed by Roberts's photographs. It was then seen to be a gigantic spiral or whirlpool, the rifts noticed by Bond being the lines of separation between the huge whorls of the spiral. Of course, small instruments are powerless to reveal anything of this wonderful structure; still there is an interest in being able to see, however imperfectly, an object which seems to present to our eyes the embodiment of that process by which some assume that our own system may have been shaped. So far as the powers of the best telescopes go, the Andromeda Nebula presents no appearance of stellar constitution. Its spectrum, according to Scheiner, is continuous, which would imply that in spite of appearances it is in reality composed of stars; but Sir William Huggins has seen also bright lines in it. Possibly it may represent a stage intermediate between the stellar and the gaseous.
North.
