328 PROCEEDINGS OF THE AMERICAN ACADEMY 



" 9^- 21™-. The dark spot increases, and is now seen on the satellite. 



" 9^- SB™". The spot has become conspicuous. 



" At 9''- 40'"-, the diameter of it was measured with the spider-line 

 micrometer, in the direction of the belts of Jupiter, and was found to 

 be 0."50 by B.2, immediately after the angle of position of what was 

 considered to be the longer axis = 170°. 



" The following diameters were then measured in the first position : — 



" lO**' S""'. It appears perfectly black and nearly round ; tried dif- 

 ferent powers ; it is best seen with 400 ; there are doubts of the spot 

 being round, but could not decide on any other form. 



" lO''- 32'"- Bi thinks it is not so black as it was. 



" lO*"' SS"'. Satellite past the middle, and keeps in the bright chan- 

 nel. 



" IP- 7™'. The satellite now appears black; it has accomplished 

 three quarters of its journey across the disk. 



"iP- 17™-. Spot dark as ever, perhaps darker. Bj thinks it in- 

 clines to the south-following limb of the satellite. 



" 1 1*"- 37™-. The satellite is seen broad, but not so dark ; it is getting 

 near the edge. 



" ll*"- 52™-. Can just discern the spot, but the altitude is getting 

 low ; however, the seeing is remarkably good, at times, for so low an 

 altitude. It is now doubtful whether the bright part of the satellite 

 can be seen or not, 



" ll*"- 58™. Neither spot nor satellite is visible, absolutely. 



" 12''- 6™'. One half the satellite is seen as it passes off the disk ; it 

 is bright. 



" 12''' 12™'. The last external contact was obsefved by B2, who 

 noticed that the limb of Jupiter appeared flattened. 



" 12''' 21"-. The third satellite is now seen at a considerable dis- 

 tance from Jupiter, and the order of brightness is, — first, second, 

 third. The fourth satellite is under eclipse. 



" It is evident that the third satellite is not now one half the bright- 

 ness of the second, which it far surpassed before the transit took place. 



