NATURE 



361 



THURSDAY, AUGUST 18, 1898. 



THE CORRESPONDENCE OF HUYGENS. 

 CEuvres completes de Christiaan Huygens publiies par la 

 SociM Hollandaise des Sciences. Tome Septihne. 

 Correspondence 1670-1675. Pp. 624. 410. (La Haye, 

 1897.) 



SEVEN large quarto volumes of letters to and from 

 Huygens have now been published ; but the com- 

 pletion of the work is not yet in sight, as the volume 

 before us only reaches the end of the year 1675, and 

 Huygens lived till 1695. We may therefore prob- 

 ably look forward to three or four more volumes, 

 making in all ten or eleven, before this undertaking is 

 brought to a close. A future historian of science in the 

 seventeenth century will no doubt find excellent material 

 in this vast collection of letters exchanged between 

 Huygens and the principal physicists, astronomers, and 

 mathematicians of his time, to which are added many 

 short papers, reprinted from the Journal des Savants 

 and the Phil. Trans. But, on the other hand, the task 

 of the historian would have been materially lightened 

 if he had been spared the trouble of wading through a 

 great many uninteresting, more or less private, letters, 

 which help to swell these bulky volumes, but which 

 might very well have been omitted. This is particularly 

 the case with the letters written to Lodewijk Huygens, 

 for though they bear witness to the brotherly affection of 

 the writer, and are often of interest as throwing light on 

 the state of the Netherlands in the days of William III., 

 particularly in the year 1672, when the armies of Louis 

 XIV. overran the country, and the last days of the 

 Republic seemed to have come, still most of these letters 

 are rather out of place among the scientific ones, and 

 would have been better published separately. But hero- 

 worship is unfortunately a disease which it is extremely 

 difficult to resist, and we can well understand that the 

 Dutch Society of Science has wished to do honour to 

 their great countryman by giving as complete a picture 

 of him as possible, both as a private man and as a 

 philosopher. 



The years covered by the present volume, 1670- 

 1675, were by Huygens spent in Paris, where he 

 had resided since 1666, except the period from the 

 summer of 1670 till the following spring, which he 

 spent in his native country in order to recover his 

 health after a severe illness in the beginning of 1670- 

 It was a stirring time in the scientific world. The 

 discovery of the solar spectrum by Newton and the 

 method of drawing tangents to curves discovered by 

 Sluse were published in 1672, the " Horologium Oscilla- 

 torium"of Huygens was issued in 1673, giving to the 

 world the theory of the pendulum, the discovery of 

 evolutes, the isochronism of the cycloid and other 

 problems of importance ; while the application of a 

 spiral spring to the balance of a watch was first an- 

 nounced in 1675. These and other matters are discussed 

 in the correspondence ; while the great respect in which 

 Huygens was held is also shown by letters on other 

 subjects, on which his opinion was asked. Thus the 

 architect Perrault, the builder of the palatial Paris 

 NO. 1503, VOL. 58] 



Observatory, " le plus somptueux monument qu'on a 

 jamais consacr^ k I'astronomie," as Lalande calls it, 

 sends Huygens a long essay giving his ideas about the 

 origin of springs in the earth ; it forms the preface to 

 his " Traitd de I'Origine des Fontaines," and need 

 therefore not have been inserted among the corre- 

 spondence of Huygens, as the reply of the latter, in 

 which he shortly gives the theory of the barometer and 

 the syphon, can be read without reference to Perrault's 

 essay. We also find Huygens consulted on matters 

 more outside his own sphere ; thus he and Hudde in 

 1671, at the request of the States of Holland and West 

 Frisland, sent a lengthy report to the States-General on 

 the deepening and regulation of the Lower Rhine and 

 the Yssel, on which subject Huygens and Hudde also 

 exchanged several letters. 



There are not many letters in this volume on practical 

 astronomy, for the simple reason that most practical 

 astronomers at that time lived in Paris ; Cassini, Picard 

 and Roemer were there ; in England, Flamsteed and 

 Halley were still young men, and in the rest of Europe 

 there were simply no observers except Hevelius. There 

 are, however, some letters and short papers (some of 

 which were printed at the time in the Journal des 

 Savants) on the disappearance of Saturn's ring in 167 1, 

 in which year the earth twice passed through the plane 

 of the ring and supplied a splendid confirmation of 

 Huygens' discovery of the true nature of the appendages 

 of the planet. The phenomena were carefully observed 

 both by Huygens himself and at the new Paris Observ- 

 atory by Cassini, who shortly afterwards discovered two 

 satellites of Saturn with the new telescopes constructed 

 by Campani. The excellence of these is acknowledged 

 by Huygens in a letter to his brother Constantin, in 

 which he humorously remarks that though the new 

 lenses of 36 and 46 feet focal length show mountains and 

 other surface-details on the moon much better than the 

 old ones did, we have not yet got so far as to see church 

 spires and trees. The construction of telescopes was a 

 subject in which the two brothers were both specially 

 interested, and on several occasions Christian sent 

 Constantin information about the new methods of 

 polishing lenses practised in Paris by Le Bas and Borel. 

 It is well known that the single-lens objectives of those 

 days were of very great focal length ; there was one of 

 60 feet at the Paris Observatory, which was very 

 troublesome to use, and Borel even boasted of having 

 made one of 150 feet ; "mais il est Gascon," says 

 Huygens. 



In England the desire of getting achromatic telescopes 

 had led Gregory and Newton to the invention of the re- 

 flecting telescope. In this country Huygens, who was 

 himself a Fellow of the Royal Society, had an indefatigable 

 correspondent in Oldenburg, who not only as secretary 

 to the Society and editor of the Philosophical Trans- 

 actions, but also by his very extensive correspondence, 

 was one of the chief centres of scientific life. At the 

 desire of the Society, Oldenburg communicated an 

 account of Newton's invention to Huygens, who pub- 

 lished it in \he Journal des Savants of February 29, 1672, 

 and also sent his brother Constantin a description of it. 

 He tried at once to make a mirror for himself, but found 

 great practical difficulties in getting a good pohsh with- 



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