457 
NATURE 
THURSDAY, SEPTEMBER 14, 1899. 
THE CORRESPONDENCE OF HUYGENS. 
Guzres completes de Christiaan Huygens publides par 
la Société Hollandaise des Sciences. Tome Huitiéme. 
Correspondance 1676-1684. Pp. 629. 4to. (La Haye: 
Nijhoff, 1899). 
| Eb esate a year after the seventh volume of 
Huygens’ correspondence the eighth one has 
made its appearance. As it embraces nine years, and 
Huygens only lived ten years longer, we may expect that 
the ninth volume will be the last one devoted to his 
correspondence. The objection which we made a year 
ago to the many comparatively uninteresting private 
letters which have contributed so much to the huge 
extent to which this collection has grown, applies equally 
to the present volume. No doubt Huygens was very 
glad to get the pleasant, gossiping letters from his sister 
Susanna, who kept him up to what was going on in 
Holland during his stay in Paris, but posterity will hardly 
be equally grateful to the far too conscientious editors 
who have considered it a duty to insert them among 
Huygens’ “ GEuvres completes.” One feels momentarily 
almost kindly disposed towards the niece of Peiresc, 
who ruthlessly destroyed ten thousand letters found after 
him. 
Having suffered from ill-health for some months, 
Huygens left Paris for the Hague in the summer of 
1676. In the autumn of the following year he wrote to 
Colbert to apologise for not coming back that winter 
owing to his health, but it appears from a letter to his 
brother Constantin that this was a mere excuse. To his 
absence from Paris at this particular time we owe some 
very interesting letters, exchanged between him and 
Roemer about the latter’s discovery of the gradual pro- 
pagation of light. Huygens had to return to Paris in 
the summer of 1678, as he did not wish to lose his French 
pension, but in 1681 he bade a final farewell to Paris and 
established himself in his native country. It has been 
generally supposed that he and Roemer (who had left 
the French capital a few months before him) were 
induced to do so by the feeling that Protestants were 
about to have a bad time in France, though as a matter 
of fact the edict of Nantes was not repealed till four 
years later. No doubt this feeling may have had some- 
thing to do with their departure, but we learn from his 
correspondence that Huygens did not debar himself 
from returning, but kept a door open by writing from 
time to time to Colbert, and after his death to Louvois, 
regretting that the necessity of being near skilful work- 
men prevented him from returning yet, and expressing 
the hope that his pension might not be finally withdrawn. 
Doubtless he spoke his mind more honestly when he 
wrote to Constantin (September 1682) that he had no 
intention of living in France, partly on account of the 
three illnesses he had suffered from there, “and also for 
other reasons,” but that he wanted to try to get some 
part of his pension without living in France. From the 
very last letter in the volume we see that he was in 
December 1684 still pegging away at Louvois, but the 
Minister of War of Louis XIV. no doubt considered that 
NO. 1559, VOL. 60] 
money was too scarce to be spent on a foreign philo- 
sopher. 
The skilful workmen, whom Huygens wanted in 1681 
were required for the completion of his planetary machine, 
by which he claimed to represent the motions in the 
solar system with considerable accuracy. Huygens was 
very proud of the performance of this machine, which is 
still preserved at the Leiden Observatory, and in letters 
to Colbert and others he repeatedly lays stress on its 
superiority to the machine for the same purpose con- 
structed by Roemer, which is also still in existence, in the 
“Round Tower” at Copenhagen, on the top of which the 
observatory was formerly situated. While working at 
this machine Huygens also wrote to Paris that he was. 
engaged in the perfection of time-keepers for finding the- 
longitude, and that he did so at the instance of the East 
India Company, but we hear nothing further about this. 
matter. After his return to Holland he and Constantin 
resumed their investigations on the best methods of 
polishing lenses for telescopes, and in a number of letters 
they exchanged their ideas as to the proper construction 
of polishing machines, &c. In 1684 Christian Huygens. 
published his “ Astroscopia compendiaria,” in which he 
described his method of using very long telescopes with- 
out tubes, keeping the eye-piece in the optical axis of the 
object-glass by means of a long string which connected 
two rods attached one to each. Ina letter dated June 5,, 
1684, J. D. Cassini makes the very remarkable suggestion 
that the object-glass might be moved by a monster clock 
moving in the plane of the equator, on which for the 
hand was substituted a perpendicular plane to which the 
lens might be attached according to the declination of 
the star. We believe this to be the earliest suggestion of 
an equatorial moved by clockwork. On the other hand, 
Perrault, a month later, sent Huygens a design of a hori- 
zontal telescope into which the light from the star was 
thrown by a mirror kept in the proper position by an 
assistant, who pointed to the star with a small altazimuth 
tube connected with the mirror by a system of pulleys. 
He also sent a similar design by a certain Boffat. It is 
interesting to see that the horizontal telescope has been: 
proposed so long ago. 
During the period (1676-84) covered by the present 
volume several discoveries of the highest importance 
were given to the world, especially the discovery of the 
differential calculus by Leibnitz, but the communications, 
exchanged between him and Huygens on this subject 
have already been printed in the collections of Uylen- 
broek and Gerhardt. Quite new, on the other hand, are 
the letters on Roemer’s discovery of the gradual pro- 
pagation of light, only the two first of which have been 
printed before (in Horrebow’s “Opera mathematico- 
physica,” T. iil. pp. 126-127, apparently not noticed by 
the editors). In reply to an inquiry from Huygens, who 
had seen in the Phz/. Trans., No. 136, a short account 
of Roemer’s paper (laid before the Academy on November 
22, 1676), Roemer informed him, under date September 
30, 1677, that Picard acknowledged the reality of the 
discovery, but that Cassini did not, as only the first 
satellite of Jupiter showed the phenomenon. Roemer 
explained this by pointing out that the occultations of 
the outer satellites were less frequent, the moments of 
their occultations less sharply observable owing to the: 
Xx 
