We now come to the best known planet of the solar system, reckoning outwards from the Sun, and though this planet itself, as an object to look at, has no particular interest for the general public, yet the history of its discovery is a matter of extreme interest. Moreover, it is very closely mixed up with the history of the planet Uranus, which has just been described. After Uranus had become fully recognised as a regular member of the solar system, a French astronomer named Alexis Bouvard set himself the task of exhaustively considering the movements of Uranus with a view of determining its orbit with the utmost possible exactness. His available materials ranged themselves in two groups:—the modern observations between 1781 and 1820, and the early observations of Flamsteed, Bradley, Mayer, and Le Monnier, extending from 1690 to 1771. Bouvard found in substance that he could frame an orbit which would fit in with each group of observations, but that he could not obtain an orbit which would reconcile both sets of observations during the 130 years over which they jointly extended. He therefore rashly came to the conclusion that the earlier observations, having been made when methods and instruments were alike relatively imperfect, were probably inaccurate or otherwise untrustworthy, and had better be rejected. This seemed for awhile to solve the difficulty, and results which he published in 1821 represented with all reasonable accuracy the then movements of the planet. A very few years, however, sufficed to reveal discordances between observation and theory, so marked and regular as to make it perfectly clear that it was not Bouvard’s work which was faulty but that Uranus itself had gone astray through the operation of definite but as yet unknown causes. What these causes were could only be a matter of surmise based upon the evident fact that there was some source of disturbance which was evidently throwing Uranus out of its proper place and regular course. First one and then another astronomer gave attention and thought to the matter, and eventually the conclusion was arrived at that there existed, more remote from the Sun than Uranus, an undiscovered planet which was able to make its influence felt by deranging the movements of Uranus in its ordinary journey round the Sun every 84 years. This conclusion on the part of astronomers becoming known, a young Cambridge student, then at St. John’s College, John Couch Adams by name, resolved, in July 1841, to take up the subject, though it was not until 1843 that he actually did so. The problem to be solved was to suggest the precise place in the sky at a given time of an imaginary planet massive enough to push, or pull, out of its normal place the planet Uranus, which was evidently being pushed at one time and pulled at another. It would also be part of the problem to predict the distance from the Sun of the planet thus imagined to exist. Adams worked patiently and silently at this very profound and difficult problem for 1¾ years when he found himself able to forward to Airy, who had become Astronomer Royal after being a Cambridge Professor, some provisional elements of an imaginary planet of a size, at a distance, and in a position to meet the circumstances. It is greatly to be regretted, on more grounds than one, that Airy did nothing but pigeon-hole Adams’s papers. Had he done what might have been, and probably was, expected, that is, had he made them public, or better still had he made telescopic use of them, a long and unpleasant international controversy would have been avoided, and Adams would not have been robbed in part of the well-deserved fruits of his protracted labours.

We must now turn to consider something that was happening in France. In the summer of 1845, just before Adams had finished his work, and one and a half years after he commenced it a young Frenchman, who afterwards rose to great eminence, U. J. J. Le Verrier, turned his attention to the movements of Uranus with a view of ascertaining the cause of their recognised irregularity. In November 1845 he made public the conclusion that those irregularities did not exclusively depend upon Jupiter or Saturn. He followed this up in June 1846 by a second memoir to prove that an unknown exterior planet was the cause of all the trouble, and he assigned evidence as to its position very much as Adams had done 8 months previously. Airy on receiving a copy of Le Verrier’s memoir seems so far, at last, to have been roused that he took the trouble to compare Le Verrier’s conclusions with those of Adams so long in his possession neglected. Finding that a remarkably close accord existed between the conclusions of the two men, he came to realise that both must be of value, and he wrote a fortnight later to suggest to Professor Challis the desirability of his instituting a search for the suspected planet. Challis began within two days, but was handicapped by not having in his possession any map of the stars in the neighbourhood suggested as the locale of the planet. He lost no time however in making such a map, but, of course, the doing so caused an appreciable delay, and it was not until September 29, 1846, that he found an object which excited his suspicions and eventually proved to be the planet sought for. It was subsequently ascertained that the planet had been recorded as a star on August 4 and 12, and that the star of August 12 was missing from the zone observed on July 30. The discovery of the planet was therefore just missed on August 12 because the results of each evening’s work were not adequately compared with what had gone before.

Meanwhile things had not been standing still in France. In August 1846, Le Verrier published a third memoir intended to develope information respecting the probable position of the planet in the heavens. In September 23 a summary of this third memoir was received by Encke at Berlin, accompanied by the request that he would cooperate instrumentally in the search for it. Encke at once directed two of his assistants named D’Arrest and Galle to do this, and they were fortunately well circumstanced for the task. Unlike Challis, who, as we have seen, could do nothing until he had made a map for himself, the Berlin observers had one ready to hand, which by good chance had just been published by the Berlin Academy for the part of the heavens which both Adams and Le Verrier assigned as the probable locality in which the anxiously desired planet would be found. Galle called out the visible stars one by one whilst D’Arrest checked them by the map, and suddenly he came upon an unmarked object which at the moment looked like an 8th magnitude star. The following night showed that the suspicious object was in motion, and it was soon ascertained to be the trans-Uranian planet which was being searched for. The discovery when announced excited the liveliest interest all over the world. It did more; it created a bitter feeling of resentment on the part of French astronomers that the laurels claimed by them should have been also claimed in an equal share by a young and unknown Englishman, and accordingly the old cry of “perfide Albion” arose on all sides. I have been particular in stating the various dates which belong to this narrative, in order to make as clear as possible the facts of the case. This is even now necessary, because though the astronomers of England and Germany are willing to give Adams and Le Verrier each their fair share of this great discovery, the same impartial spirit is not to be found in France, for nothing is more common, even in the present day, in looking at French books of astronomy, than to find Adams’s name either glossed over or absolutely suppressed altogether when the planet Neptune is under discussion.

How remarkable a discovery this was, will perhaps be realized, when it is stated that Adams was only 2½° out in assigning the position of the new planet, whilst Le Verrier was even nearer, being barely 1° out.

We know practically nothing respecting the physical appearance of Neptune, owing to its immense distance from us, and for the like reason the Neptunian astronomers, if there are any, will know absolutely nothing about the Earth; indeed, their knowledge of the Solar System will be restricted to Uranus, Saturn, and the Sun. Even the Sun will only have an apparent diameter of about 1' of arc, and, therefore, will only seem to be a very bright star, yielding light equal in amount, according to ZÖllner, to about 700 full moons. There is one satellite belonging to Neptune, and as this has been calculated to exhibit a disc 10° in diameter, a certain amount of light will no doubt be afforded by it especially if, as is not unlikely, Neptune is itself possessed of some inherent luminosity independently of the Sun.

The fact that Neptune seems destitute of visible spots or belts, results in our being ignorant of the period of its axial rotation, though it should be stated that in 1883, Maxwell Hall in Jamaica, observed periodical fluctuations in its light, which he thought implied that the planet rotated on its axis in rather less than 8 hours. Several observers thought 20 or 30 years ago, that they had noticed indications of Neptune being surrounded by a ring like Saturn’s ring, but the evidence as to this is very inconclusive. It is quite certain that none but the very largest telescopes in the world would show any such appendage, and this planet seems to have been neglected of late years, by the possessors of such telescopes. Moreover, if a ring existed it would only open out to its full extent once in every 82 years, being the half of the period of the planet’s revolution round the Sun (just as Saturn’s ring only opens out to the fullest extent every 14½ years), so that, obviously, supposing suspicions of a ring dating back 30 or 40 years were well founded, it might well be that another 30 or 40 years might need to elapse, before astronomers would be in a position to see their suspicions revive.

Neptune revolves round the Sun in 164½ years, at a mean distance of 2791 millions of miles. Its apparent diameter scarcely varies from 2¾. Its true diameter is about 37,000 miles. No compression of the Poles is perceptible. Its one satellite revolves round Neptune in 5¾ days, and in a retrograde direction, at a mean distance of 223,000 miles, and shines as a star of the 14th magnitude. This is a peculiarity which it only shares with the satellites of Uranus, so far as it regards the planetary members of the Solar System, though there are many retrograde Comets.