520 lioyal Astronomical Society. 



" The fact of these inequalities always retaining one unaltered po- 

 sition may be explained without precluding the possibility of a rajnd 

 rotation of the rings, by attributing them to the reflexion of the solar 

 rays from their inner edges. There seems to be no other way of ac- 

 counting for their being seen on both surfaces, both when illuminated 

 and when turned away from the sun. 



" The first reappearance of the ring took place between August 

 31^'78, Greenwich M.T., and Sept. 3^-80; the second disappearance 

 between Sept. 12'*-80 and Sept 13'^-62 ; and the final reappearance 

 in 1849 between Jan. 18'i-47 and Jan. I9'^-43. 



" On two or three occasions in the past year we have enjoyed the 

 sight of all the eight satellites at once. 



"Our observations for a year or two past on the variations of the 

 brightness of Jupiter's satellites afford some curious results. Atten- 

 tion was first drawn to the subject in observing a transit of the third 

 satellite on the 6th of January, 1847. A few minutes after its en- 

 trance the satellite was visible on the disc just within the limit of 

 the planet, being brighter than the surrounding surface ; soon after 

 a dark spot, supposed to be the satellite, could just be discerned in 

 its place. The telescope used was an achromatic of 2|- inches aper- 

 ture, of excellent quality, but not powerful enough to enable us to 

 decide whether or not the spot was really the satellite. 



" On the 28th of January, 1848, during the transit of the shadows 

 of the first and third satellites, the third satellite itself was seen with 

 the great refractor under very beautiful definition, as a black spot 

 between the two shadows, and not to be distinguished from them 

 except by the 2)lace it occupied. It was smaller than its shadow in 

 the proportion of 3 to 5, not duskish simply, but quite black like the 

 shadows. On the 11th of March it was again seen dark on the disc, 

 smaller than its shadow. 



" On the 18th of March we watched the entire transit. At the 

 first internal contact the satellite -was distinctly seen on the disc, 

 brighter than Jupiter, though it had entered on a bright channel 

 south of one of the great equatoreal belts ; 20 minutes after it had 

 become nearly of the same brightness with the planet, so as to be 

 barely perceptible, yet still whiter than the surrounding surface. 

 While watching it with close attention, a minute dark speck sud- 

 denly made its appearance, in the place of the satellite, increasing 

 very rapidly till it occupied a space of about one second of arc in 

 diameter, quite black and nearly round, though an irregularity of 

 shape was suspected. Remaining thus for about two hours, tlie dark- 

 ness gradually lost its intensity, and quite disappeared before the 

 satellite left the disc. Something of this nature we have always ob- 

 served to accompany a transit of this satellite. The first and fourth 

 satellites we have also seen black or dusky on the disc, but the former 

 has once or twice crossed without our detecting any change. The 

 spots are always less than the shadows,but have appreciable diameters, 

 and make their appearance after the entrance of the satellite upon 

 the limb of Jupiter. Changes of relative brightness are constantly 

 going on ; the feeblest, on the average, being the fourth, and the 

 brightest usually the third. 



