232 The Planets of the Month 



to possess the means of witnessing the minuter details for 

 themselves : — " 1848, Jan. 28. — The 3rd satellite itself was 

 seen with the great refractor" (of nearly fifteen inches aperture) 

 " under very beautiful definition, as a black spot between the 

 two shadows" (of I. and III.), " and not to be distinguished 

 from them except by the place it occupied. It was smaller 

 than its shadow in the proportion of 3 to 5, not duskish simply, 

 but quite black like the shadows;" and under March 18th, he 

 thus records a similar transit of the same satellite : — " 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 equatorial belts; twenty 

 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 suddenly made its appearance 

 in the place of the satellite, increasing very rapidly till it occu- 

 pied a space of about one second of arc in diameter, quite black, 

 and nearly round, though an irregularity of shape was suspected. 

 Eemaining thus for about two hours, the darkness gradually 

 lost its intensity, and quite disappeared before the satellite left 

 the disc." The change thus described is of course due to the 

 rapid diminution of Jupiter's light towards the edges of the 

 disc ; but it is strange that the eye is so little sensible of that 

 decrease in any other way, though its amount must be extremely 

 great, before it could thus turn white to black, from the mere 

 effect of contrast. This wonderful observation, for such it 

 really is, and will the more appear so, the more it is studied, 

 tends greatly, it must be owned, to shake our confidence in 

 the discriminating power of the eye. Dawes, from the perfec- 

 tion of whose telescopic vision there can be no appeal, informs 

 us that the 2nd satellite is never thus shaded, that I. is some- 

 times grey, III. darker by many shades, and IV. darkest of all. 

 Another marvellous peculiarity, mentioned by several ob- 

 servers, is thus described by Lassell in the Monthly Notices ot 

 the Royal Astronomical Society, under the date of February 

 28th, 1850. "12k. 11m. The shadow and satellite" (IV.) 

 " being now equally distant from the limb, the greater size of 

 the shadow is most obvious ; indeed, it appears to be twice as 

 great in diameter." It is difficult to conceive any mode of ac- 

 counting for this, unless we could suppose the satellite to be 

 encompassed with an atmosphere possessed of refractive power 

 in the opposite direction to every substance or medium within 

 the compass of our knowledge ; that it was no illusion is ren- 

 dered probable by the fact that the irradiation, or spreading out 

 of light, which is inseparable from telescopic vision, would have 

 produced exactly the contrary effect, expanding the apparent 



