AN article on " Le tricentenairede I'abbe Picard," 

 by M. E. Doublet, in the Revue generale 

 des Sciences (September 15-30J, directs atten- 

 tion to the tercentenary of the well-known Erench 

 astronomer, Jean Picard, who was born on 

 July 21, 1620. Very little is known about 

 Picard 's life, so that even the year of his death 

 is uncertain (about 1683). He was a pupil of 

 Gassendi, and took up the study of astronomy at 

 latest in 1645, when he observed the eclipse of the 

 sun on August 21 of that year, and it was as an 

 observer that he was chiefly distinguished. 

 Though he was not the first to apply telescopic 

 sights to astronomical instruments, he was almost 

 certainly not aware that this had many years 

 before been done by William Gascoigne ; but 

 Picard was at any rate the first to make use of 

 this invention in an extensive series of observa- 

 tions, when he, in 1669 and 1670, determined the 

 size of the earth. This was done by a triangula- 

 tion from Sourdon, near Amiens, to Malvoisine, 

 south of Paris, on the plan first proposed and 

 carried out by Snellius about fifty years earlier. 

 Picard measured a base along a level and well- 

 built road from \'illejuive to Juvissy, 5663 toises 

 long. It is deserving of special notice that he 

 compared his standard toise with the length of 

 the seconds pendulum, "lest the same should 

 happen to it as had happened to all previous 

 standards " ; and that did indeed happen, for his 

 toise is lost. The results of this, the first modern 

 geodetic operation, were p^iblished in Picard's 

 "Mesure de la terre " in 1671. 



In 1669 Picard presented to the Academy a 

 memoir on the most important astronomical ob- 

 servations which ought to be undertaken. Among 

 these is a new determination of the right ascen- 

 sions of stars by direct comparison with the sun ; 

 this had never been done before without observing 

 an intermediary body (the moon or Venus) which 

 could be seen in the daytime. But it was now 

 possible, as Picard had found in the previous 

 NO. 2663, VOL. 106] 



year, to observe stars in daylight with the 

 telescope attached to his quadrant. Another 

 desirable undertaking was the accurate deter- 

 mination of the position of Tycho Brahe's observa- 

 tory. This he was able to carry out in 1671, 

 when the Academy sent him to Denmark for that 

 purpose. The results of his observations on the 

 site of Uraniborg were published in 1680 in his 

 "Voyage d'Uranibourg." This expedition became 

 memorable in two ways. First, because Picard,. 

 in his account of it, describes certain apparent 

 motions of the Pole-star towards or away from 

 the pole, of which the period was a year, and 

 which, he says, he had noticed for about ten years. 

 From the details given, it is evident that Picard 

 was the first to notice the effect of aberration on 

 the apparent place of a fixed star ; and when he 

 adds that these irregularities were in some years 

 smaller than in others, it shows that the effect of 

 nutation was also beginning to be felt. But it 

 was reserved for Bradley both to discover the 

 laws governing these phenomena and to give the 

 correct explanation of them. 



The second valuable result of Picard's journey 

 to Denmark was that he made the acquaintance 

 of Romer at Copenhagen, and persuaded him to 

 go to Paris with him. Romer stayed nine years 

 in Paris, and it was there that he in 1675 an-^ 

 nounced his discovery of the gradual propagation 

 of light. We know from his letters to Huygens- 

 that he at once realised that this must produce 

 aberration. Considering that he and Picard lived 

 together at the Paris Observatory, it is rather 

 strange that they did not compare notes and 

 remark how perfectly this agreed with Picard's 

 observations of the Pole-star. But Romer scarcely 

 ever published anything, so it is not impossible 

 that he may have noticed the agreement, and did 

 not care to publish it. 



I Picard from time to time carried out various 

 geographical operations in France, the results of 



j which are included in his "Ouvrages de Mathe- 



