1869.] Astronomy. 259 



made by Mr. Lockyer and M. Janssen, of a mode of rendering the 

 prominences sensible (not visible) turns on tbe same feature." 



Mr. S. Newcomb discusses Mr. Stone's rediscussion of the 

 transit of Venus in 1769. He endeavours to show that Chappe 

 cannot be assumed to have observed an apparent internal contact of 

 Venus instead of the real one, or " the formation of the black drop " 

 as it is termed. Certainly, Chappe's words are not clear. After 

 describing the undulating appearance of both the sun and Venus, 

 he goes on to say, — "A ce premier contact Venus s'est allongee 

 j)lus considerablement que le matin, en s'approchant tout-a-coup du 

 bord du soleil." One might interpret this to signify that the 

 elongation of the disc (the pear-shaped figure it assumes) was more 

 remarkable at the egress than at the ingress; or that the sudden 

 leap, so to speak, by which the edge of the planet's disc is carried 

 forward to that of the sun's, covered a greater range than the 

 corresponding jDhenomenon (reversed) at ingress. Mr. Stone takes 

 the passage to signify that the elongation was of a more complete 

 kind, so that a hroader (not a longer) connecting-band was seen ; 

 in other words, Mr. Stone holds that Chappe missed the formation 

 of the narrow hgament owing to atmospheric undulations, and first 

 saw some phase much nearer to the apparent internal contact. 

 Weighed by results, Mr. Stone's view has clearly the advantage 

 over Mr. Newcomb's. As Mr. Stone remarks, " If the observer's 

 words are doubtful, and we find that by assuming he observed an 

 apparent contact all the observations are rendered accordant, whilst 

 by assuming that he has observed a real contact all the observations 

 are rendered discordant, it is not only permissible but necessary 

 that we should assume that apparent contacts were observed. 



Mr. Stone draws from the results of the observations made at 

 Greenwich upon the transit of Mercury in November last, that 

 the three following points should be carefully attended to in future 

 observations of the more important transits of Venus : — 



1. That telescopes of nearly the same aperture should be em- 

 ployed. 



2. That very nearly the same magnifying powers should be 

 employed by all the observers. 



3. That attention should be directed to observations of real 

 internal contacts as the chief points. 



The words " real internal contacts," it is to be observed, refer 

 to the formation of the black drop, which is undoubtedly simulta- 

 neous with the true internal contact of the planet's limb with 

 the sun's. 



Mr. Stone has extended to 1867 the calculations he had made 

 respecting the constant of nutation, as determined from the ob- 

 servations of the pole-star with the transit-circle of the Greenwich 

 Observatory from 1851 to 1865. Taking these observations, thus 



