162 RECORD OF SCIENCE FOR 18.S7 AND 1888. 



by tlie observations of Schiaparelli, Terby, Niesteii, aud Hoklen. The 

 observations of Professor lloldeu and his assistants witli the 36 inch 

 refractor began on July 10, 18S8, and were continued to August 10; 

 the planet was therefore very unfavorably situated, its diameter being 

 less than 9". Several of the most important canals were seen, but thcv 

 were not double, appearing rather "as broad bands covering the spacea 

 on M. Schiaparelli's map which are occupied by pairs of canals and by 

 the spaces separating the members of each i)air." Professor Hall, with 

 the Washington 20 inch refractor, has never been able to see these 

 markings so sharply drawn by European observers. The only remark- 

 able change he noticed was the dimiuution in the size of the white spot 

 at the south pole of the planet. 



Numerous sketches of Mars showing the canals or other markin-gs 

 have- been published by llolden, Perrotin, Terby, and Niesten. No ad- 

 equate explanation of the canals, or of the changes observed, has yet 

 been offered. 



Jupiter's satellites. — Astronomers have always been puzzled by 

 the discordant appearances of the satellites during transit^ but more 

 especially by the fact that the phenomena do not apply equally to all the 

 satellites, or even in some instances to the same satellite in two succes- 

 sive revolutions. The fourth, for instance, as it approaches the disk of 

 Jui)iter becomes rapidly fainter till it arrives at contact. When once 

 on the hmb it shines with a moderate brilliancy lor about ten or fifteen 

 minutes, then becomes suddenly lost to view for a siniilar period, and 

 lastly reappears, but as a dark spot, which grows darker and darker 

 until it equals the blackness of its ow^n shadow on the planet. The ap- 

 pearance of the second satellite, however, is entirely different, for it 

 seems never to have been seen otherwise than pure white during transit; 

 whereas the first and third differ yet again from the preceding two. 

 The former is sometimes a steel-gray, and at others a little darker, 

 whereas the latter has been seen perfectly white and yet so black as to 

 be mistaken for the fourth. Mr. E. J. Spitta has made a careful investi- 

 gation of these interesting phenomena, communicating his results in a 

 paper of some length, read at the meeting of the lloyal Astronomical 

 Society, in November, 1887. His experiments consisted essentially of 

 numerous observations ui)on suitably pre[)ared models representing the 

 planet and satellites, and he concludes (see Nature 37:J:08, Marcli 15, 

 1888) that the probable reason th€ fourth satellite is uniformly black 

 during trausit, when it has i)assed its period of disappearance, is, that 

 its albedo is so low as to grant the ditference between it and the back- 

 ground necessary for a body to ai)pear black when superimposed on an- 

 other. Its preliminary whiteness and disappearance are also shown to 

 be a question of relative albedo, for they are due to the fact that a sphere 

 at its limb loses so much in reflective power that up to that moment 

 the satellite possesses sufficient albedo (as compared with the back- 

 ground in that situation Uo maintain its whiteness. So, too, with the sec- 



