published till after the death of Huygens, was chiefly a treatise on the habitability of other worlds than ours, and was marked by curious and ingenious speculations, of a character from which his other works were almost entirely free. In this work, after expressing his belief in the existence upon the planets of living bodies in no way inferior to those on the earth, he added: "What obliges me to believe also that there is a rational animal in the planets is that, if there is not, the earth would have too great advantages (while it is one of the smallest of the planets) and would be too much elevated in dignity (while it is neither the nearest to the sun nor the most distant from it) over the other planets, if it had an animal so much superior to all that they have. . . . Finally, is it reasonable to suppose that the heavenly bodies among which our earth occupies so modest a rank have been created only in order that we other little men may enjoy their light and contemplate their situation and motion?" He also gave some vivid pictures of the scenery of the heavens as observed from the different planets, paraphrases of which had wide circulation in an English work of popular astronomy of the last generation. In observing the moon he made a study of its mountains and plains, and, remarking that the latter were too rough to be lakes or oceans, concluded, what is now generally believed, that the moon has no bodies of water; also that it has no atmosphere—none at least that rises above the valleys.
At the beginning of the year 1695, Huygens lost his faculties—an affliction he had suffered once before while residing in Paris, but from which he had recovered after removal to his native land. This time the affliction was permanent, except for a few lucid intervals which he employed in making testamentary dispositions of his property, and in consigning the care of his manuscripts to his friends Bürcher de Volder and Bernard Fullen.
Like his illustrious contemporaries Descartes, Leibnitz, and Newton, Huygens was never married. He is described as having had a good figure, and been possessed of a noble and elevated character. He was affable and frank in his disposition, and gave a warm welcome to inquiring young men, whom he was always ready to direct in the way of discovery. It was thus that Leibnitz came to him and received the inspiration of which we have quoted the acknowledgment. Though qualified by birth and fortune to shine in society, and constrained to figure there for a part of his life, he preferred retreat, and passed all of his time that he could in the country, immersed in his studies and experiments.