THE fourth volume of the Huygens correspondence, covering the years 1662-63, is now before us. Although the interval, as regards fresh discoveries by the “Dutch Archimedes,” was a comparatively barren one, the 249 letters referable to it (to say nothing of supplementary documents) afford materials for much instruction, and some entertainment. It is much to learn that the greatest astronomer of his time sought to keep in touch with Paris in respect to the cut and colour of his clothes; nor can we be indifferent as to the precise date of his beginning to wear a wig. On June 15, 1662, at the age of thirty-four, he communicated to his brother Constantine the distressing intelligence of his incipient baldness; a remedy for which, in the shape of the best perruque to be had in Paris, was provided in the following October. A similar article of attire, despatched by him to the Hague for his elder brother's wear, figures in several letters, and engaged many anxious thoughts; but a little plot concocted by the par nobile fratrum for extracting the price—amounting to four and a half louis—from the liberality of their father, the Secretary, appears to have been baffled. They were, indeed, often made to feel—though not with any unreasonable harshness—that he who keeps the purse holds the reins; for the paternal authority exercised in their family was of no shadowy kind. The elder Constantine ordered his three sons—middle-aged, in Dante's sense, though they were—from realm to realm at his good pleasure, and was obeyed without hesitation. And notwithstanding that his demands from Paris for optical toys—pocket-telescopes, magic-lanterns, and the like—gave Christian considerable annoyance, he did not venture to refuse, or so much as remonstrate against the fulfilment of paltry, troublesome, and, to his sentiment, humiliating commissions. He, however, stooped instead to the scarcely laudable subterfuge of begging his brother Louis, then in Paris, to abstract one of the three lenses of the lantern, and so bring about at least a postponement of his father's appearance at the Louvre in the character of showman to scientific “marionettes,” no longer claiming even the distinction of novelty. Œuvres Complètes de Christiaan Huygens. Publiées par la Société Hollandaise des Sciences. Tome quatrième, Correspondance 1662-63. (La Haye: Martinus Nijhoff, 1891.)