I40 



NATURE 



[September 30, 1920 



ences, have been the chief synthetic forces 

 throughout the great story of enlarging human 

 co-operations : of the former we may look for a 

 revival, of the latter a re-adjustment informed 

 with science. As a necessary basis for world co- 

 operation, as a preparation for a world league of 

 men, there must be "a nevo telliiif^ and interpreta- 

 tion, a common interpretation, of history." And 

 that this book will further. "There can be little 

 question that the attainment of a federation of all 

 humanity, together with a sufficient measure of 

 social justice, to ensure health, education, and a 

 rough equality of opportunity to most of the chil- 

 dren born into the world, would mean such a 

 release and increase of human' energy as to open 

 a new phase in human history." Mr. Wells looks 

 forward to " the final achievement of world-wide 

 political and social unity," which will be reached 

 by and based on righteousness as well as science, 

 "perhaps with long interludes of setback and dis- 

 aster," but "it will mean no resting stage, nor 

 even a breathing stage, before the development of 

 a new struggle and of new and vaster efforts. Men 

 will unify only to intensify the search for know- 

 ledge and power and live as ever for new occa- 

 sions." Almost the ending of what we cannot but 

 regard as a great book is a key sentence : " Life 

 begins perpetually." 



The Dioptrics of Huygens. 



CEuvres Completes de Christiaan Huygens. Tome 

 Treizi&me. Dioptrique 1653; 1666; 1685-1692. 

 Fascicule i., 1653; 1666. Pp. cIxvii4-432. 

 Fascicule ii., 1685-1692. Pp. 434-905. (La 

 Haye : Martinus Nijhoff, 1916.) 



NE.'VRLY ten years have elapsed since we re- 

 viewed the twelfth volume of this great work 

 (Nature, vol. Ixxxiv., p. 491), of which vol. xiii., 

 consisting of two parts of altogether nearly 1 100 

 pages, only lately reached us, though dated 1916. 

 It contains everything written by Huygens on geo- 

 metrical optics, both what was incorporated in the 

 "Dioptrica" published in his "Opuscula Post- 

 huma " in 1703 and 1728, and various extracts 

 from his manuscripts now printed for the first 

 time. 



Already in 1652 and 1653 Huygens had written 

 a treatise on refraction and telescopes, divided 

 into three books, and by the present editors called 

 part i. In the next twelve years he occupied him- 

 self with the subject from time to time, as appears 

 from his correspondence. In 1665 he resumed 

 more systematically his researches on spherical 

 aberration, and found results which seemed to him 

 NO. 2657, VOL. 106] 



so important that he wrote an essay (part ii.) on 

 them. This was soon interrupted by his settling' 

 at Paris as a member of the Academy and a pen- 

 sioner of Louis XI\'., but within a year he had a 

 copy made of all he had yet written on the subject, 

 and this is still in existence. In 1668 Huygens 

 tried to verify by experiments his theory of 

 spherical aberration, by which he thought he would 

 be able to compensate the aberration of the object 

 glass by that of the eyepiece ; but this led only to 

 disappointment, and he perceived that it was colour- 

 effects which prevented the realisation of his ideas. 

 He next got the idea of forming the object-glass 

 of two lenses, situated close to each other, an idea 

 which was never incorporated in his MS. ; and 

 it was probably the uncertainty he felt about the 

 value of his latest results which still made him put 

 off the publication of his work. In 1672 he heard 

 of Newton's discovery of the composition of white 

 light, and after some hesitation he realised its 

 fundamental importance for the problems of 

 dioptrics, so that the results he had himself found 

 had not the value he had supposed them to have> 

 and they were therefore removed from his MS. 



In the meantime the undulatory theory of light 

 had arisen in the mind of Huygens, and he pro- 

 posed to write a larger work dealing with the new 

 theory and its applications, in which his MS. on 

 dioptrics might find a place. But in 1677 he dis- 

 covered the explanation of double refraction in 

 Iceland spar, which he considered the finest con- 

 firmation of his new theory, and beside which all 

 his earlier work on dioptrics appeared to be of 

 secondary importance. He therefore decided to let 

 this work be preceded by a treatise on the undula- 

 tory theory of light and its principal applications, 

 without dealing with the theory of lenses and tele- 

 scopes. This was the origin of the celebrated little 

 book, "Traite de la lumifere," which, though not 

 published till 1690, had been practically completed 

 in 1678. Finally, in 1684, Huygens resumed his 

 researches on the magnifying power of telescojjes 

 and questions related thereto, and it was probably 

 in the following year that he wrote nearly all that 

 which in the present edition has been put together 

 in part iii., on telescopes. In 1692 he wrote to 

 Leibniz that he seemed to have finished with the 

 subject, though everything was not yet written 

 down. He then numbered the pages of his MS. 

 in the order which he proposed to follow in the 

 final redaction. This pagination was generally 

 followed in the posthumous edition of 1703, but in 

 this way parts written at very different epochs are 

 mixed up together. The present editors therefore 

 preferred to follow the chronological order, so that 

 the gradual development of Huygens 's ideas could 



