( 312 ) 



against the real intentiop. put at 4 to 11 instead of at 4 to 7. We 

 thus get for the diameters 0"'8G3 and 1"'511, which is still in good 

 agreement with the result of Mr. Nijland. As values have been assumed 

 for the radii of the orbits which hold for the mean distance of Jupiter 

 from the sun, these values need no further reduction. 



(3) We find in Houzeau, Vademecum (Bruxelles, 1882), p. 666 : 

 On rapporte line occultation du satellite II par le satellite III, 

 observée a Sommerfeld, prés de Leipzig, par C. Arnoldt, le l^^" 

 novembre 1693, (Whiston, The longitude discovered by the eclipses, 

 8°, London, 1738), et iine autre du satellite IV, également par le 

 III'"*', vue par Luthmer k Hanovre, le 30 octobre 1822 {Nature, 4**, 

 London; vol. XVII, 1877, p. 148). 



1st Remark. The little book of Whiston here quoted is in the 

 library of the University at Utrecht, Division P, 8^», number 602. We 

 have turned over the leaves several times, but have not found any 

 mention of the observation of G. Arnoldt. It is true that the author, 

 in § XVIII, recommends the observation of the mutual occultations of 

 the satellites. He remarks that, if at such an occultation they have 

 opposite motions, the relative velocity is "doubled". He mentions the 

 complaint of Derham ^), that the strong light of Jupiter renders the 

 observation of these occultations rather difficult. He remarks that, the 

 interval being equal, their number must be one and a half time as 

 large as that of the eclipses. Again he mentions that Lynn is the 

 first who, in the Philosophical Transactions N^. 393, has proposed to 

 apply these conjunctions to the determination of the longitude, seeing that 

 they can often be observed with an accuracy of less then half a minute 2). 

 But I do not find the obseivation of a single occultation nor its prediction. 



It needs hardly be said that the conjunctions, visible from places, 

 the difference in longitude of which is to be determined, are too rare 

 to be of much importance for the purpose. In accuracy of observation 

 they are at all events surpassed by occultations of stars. But they 

 may well be compared with the eclipses of the satellites of Jupiter and 

 are indeed superior to them in this respect that they yield a result in 

 a few minutes which is independent of the optical power of the telescope. 

 For the eclipses this is only true in the case of the combination of a 

 disappearance with a reappearance. 



2nd Remark. The original account of the observation of Luthmer 

 was communicated by him to Bode who inserted it in the (Berliner) 

 Astronomisches Jahrbuch fur 1826, p. 224 : 



"Am 30 Oct. Ab. 6" 55' Bedeckung des vierten ^ Trabanten vom 

 dritten." 



1) Poggendorff's Biograpkisches Wörterbuch, (article W. Derham) gives no 

 reference to the passage where this complaint is to be found, nor even to any 

 paper on the observation of the satellites of Jupiter. 



2) At least if there were no undulation of the images. See at the end of note 4 



