162 KECOKD OF SCIENCE FOR 1887 AND 1888. 



by tlie observations of Scliiaparelli, Terby, Niesteii, and Hokleii. The 

 observations of Professor Ib>l(leu and his assistants with the oG inch 

 refractor began on Jnly IG, 1SS8, 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 spaces 

 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 26 inch refractor, has never been able to see these 

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

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

 at the sDuth pole of the planet. 



Numerous sketches of Mars showiug the canals or other markings 

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

 ecjuate «^x[»laimtiou of the canals, or of the changes observed, has yet 

 been otfered. 



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

 the discordant appearances of the satellites during transit.j but more 

 especially by the fact that the phenomena do not appW 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 limb it shines with a moderate brilliancy for about ten or fifteen 

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

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

 until it equals the blackness of its own shadow on the i)lanet. The ap- 

 pearance of tlie second satellite, however, is entirely different, tor 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 peifectly 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, 3887. His exi)eriments consisted essentially of 

 numerous observations ujjou suitably prepared models representing the 

 planet and satellites, and he concludes (see Nature 37:468, March 15, 

 1888) that the probable reason the fourth satellite is uniforndy black 

 during transit, when it has passed its period of disappearance, is, that 

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

 ground necessary for a body to a[)i)ear black when superimposed on an- 

 other. Its preliminary whiteness and disapi)earance 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 retiective power that up to that moment 

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

 ground in that situation Uo maintain its whiteness. S^), too, with the sec- 



