i873-] Changes in the Moon's Surface. 505 



compare the tint of the floor directly and solely with a 

 much brighter surface; if the comparison is made on the 

 east, with an illuminated wall, it is made on the west with 

 a perfectly black shadow-streak. Similar remarks apply to 

 the time when the terminator is about to pass away from 

 Plato. But at the time of full moon, the highlands around 

 Plato are very brightly illuminated. The glare necessarily 

 makes Plato itself look relatively dark, notwithstanding the 

 fact that the floor is also much more brilliantly illuminated ; 

 for it is a recognised fact, that surfaces of unequal light- 

 reflecting capacity appear to differ more in brightness under 

 a high illumination than when they are only faintly illu- 

 minated. We know, in fact-, that a surface which is only 

 dark looks almost or perfectly black when itself and a 

 bright background are under strong illumination. 



We have recently had some remarkable illustrations of 

 the deceptive effects of contrast in the aspect of Jupiter's 

 fourth satellite as it has crossed the face of the planet. 

 This satellite' is somewhat inferior in light-reflecting 

 capacity to the other three satellites, and if it were not for 

 the physiological law into which I am now enquiring, we 

 should expect this satellite to look somewhat darker than 

 the others when transiting the disc of Jupiter. But as a 

 matter of fact, instead of looking merely dark, this satellite 

 looks nearly black when on Jupiter's face, insomuch that it 

 has been often mistaken for a shadow of a satellite. Mr. 

 Roberts, when observing one of those dark transits of the 

 fourth satellite, could scarcely believe that the satellite 

 would be visible at all when outside the disc ; and yet, on 

 every such occasion, as soon as the transit was over, the 

 satellite was seen as usual, though, when close to the 

 planet, looking rather faint by contrast. Mr. N. E. Green, 

 a very excellent observer, and to whom we owe some 

 admirable pictures of Mars and Jupiter, made the following 

 even more pertinent observation on the 26th of March last: — 

 " The transit of the fourth satellite," he says " was closely 

 observed ; it certainly appeared as dark as any of the 

 shadows, sometimes even sooty in its blackness, and on 

 leaving the disk seemed unusually faint. But here a 

 remarkable fact, in connection with the law of contrasts, 

 was observed. No sooner had it passed away into the clear 

 sky than it seemed to be brighter than the dark belt 

 against which it had so recently appeared as a decided 

 dark." Such an observation as this appears to me to be 

 decisive against mere eye-estimations, showing that abso- 

 lutely no reliance can be placed on them unless some 



