It would appear that the elder Bouvart, the French astronomer referred to above, was the first to suggest that the disturbances in the orbit of Uranus, throwing that planet from his pathway outward, might be and probably were to be explained by the presence in outer space of an unknown ultra-Uranian planet. Bouvart prepared tables to show the perturbations in question, and declared his opinion that they were caused by an unknown planet beyond. No observer, however, undertook to verify this suggestion or to disprove it. Nor did Bouvart go so far as to indicate the particular part of the heavens which should be explored in order to find the undiscovered world. His tables, however, do show from the perturbations of the orbits of Jupiter, Saturn and Uranus that the same are caused by the mutual influence of the planets upon one another.
It seems to have remained for Dr. T.J. Hussey, of Hayes, England, to suggest the actual discovery of the unknown planet by following the clew of the disturbance produced by its presence in a certain field of space. Dr. Hussey, in 1834, wrote to Sir George Biddell Airy, astronomer royal at Greenwich, suggesting that the perturbation of the orbit of Uranus might be used as the clew for the discovery of the planet beyond. But Sir George was one of those safe, conservative scholars who scorn to follow the suggestions of genius, preferring rather to explore only what is known already. He said in answer that he doubted if the irregularity in the Uranian orbit was in such a state of demonstration as to give any hope of the discovery of the disturbing cause. He doubted even that there was such irregularity in the Uranian orbit. He was of opinion that the observers had been mistaken in the alleged detection of perturbations. So the Greenwich observatory was not used on the line of exploration suggested by Hussey.
Three years afterward, and again in 1842, Sir George received letters from the younger Bouvart, again suggesting the possibility and probability of discovering the ultra-Uranian planet. These hints were strengthened by a letter from Bessel, of Königsberg. But Sir George B. Airy refused to be led in the direction of so great a possibility.