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The present century has been remarkable for the progress which has been made in all departments of astronomy. Within the solar system, within the sidereal or galactic system, and within the yet wider range ascribed to the nebular system, discoveries of the most important character have been effected. There is a singular contrast, however between the amount of positive knowledge which has been deduced from observational labours within the solar domain, and the somewhat vague ideas which astronomers are content to hold respecting the sidereal space. I shall endeavour to exhibit the fullness of this contrast, and then to point out some of the more remarkable consequences which seem to flow from modern observations within the stellar and nebular domains.

At the end of the last century astronomers recognized in the solar system a mechanism of an uniform and symmetrical character. Around a central orb they saw revolving a family of dependent globes, vast in their absolute dimensions, but minute in comparison with the massive globe which sways their movements. Amongst these bodies they saw several attended upon by yet smaller globes, forming secondary systems, which resemble in many respects the great system of which the sun is the controlling centre. The late



discovery of Uranus had led them to recognize the possibility that beyond the known planets there may exist others, perhaps by no means the least important members of the solar system. Little was known, however, that differed in kind from what had been known to Aratus, Hipparchus, or Ptolemy. When we have named the ring of Saturn and a few periodic comets, which were looked on rather as accidental solar attendants than as forming a normal feature of the system, we have mentioned all that the three last centuries had revealed which differed in character from what had been recognized for two thousand years.

Very startling is the contrast when we turn to consider the views at present held respecting the solar domain. We no longer see a system which, however complex, might yet be very adequately represented by human mechanisms. We recognize, within a sphere exceeding manifold in diameter the orbit of distant Neptune, a variety and complexity of formation of which the human mind is unable to form adequate conceptions.

The increase in the number of primary attendants upon the Sun, though far from being the most remarkable discovery which has been made during the present century, is well worth dwelling upon for a moment. We have lately heard of the detection of the 98th asteroid, and yet it was but on the opening day of the century that the first of these bodies was discovered. In these new members of the solar system we recognize characteristics which had not hitherto been presented to the notice of astronomers. We see a series of bodies, primaries of the planetary system, which yet, instead of travelling in distinct and widely-separated orbits, revolve in paths closely interwoven. Even when but forty had been discovered it was truly said that if each orbit were represented by a hoop, it would be impossible to lift any one of these hoops without lifting the whole set. We may fairly assume that for each discovered asteroid there are to be reckoned tens, perhaps hundreds, which will remain for ever undiscovered.

It has been found, also, that there exist within the solar system myriads of dependent comets. Revolving around the Sun in orbits of the most varied figure, differing among themselves in size and character, and presenting—some of them—the most singular phenomena that have ever rewarded astronomical observation, these objects remain among the mysteries of science. The only two which have as yet been submitted to the searching analysis of the spectroscope are found to consist of a gaseous nucleus attended by a coma which probably shines by reflected light; but whether this is the case

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