210 ASTRONOMY. 



On tlie eveiiiug of February 2, Jupiter was passing near tlie star B. A. 



C. 303 (73 Piscium), and the opportunity was taken at the observatory 

 of Harvard College to compare photometrically the third satellite of the 

 planet, with the star. Three observers took part in the work, and four 

 sets of measurements, each consisting of eight single comparisons, were 

 made. The result obtained was that the star was fainter than the sat- 

 ellite hj 0.38 magnitudes of Pogson's logarithmic scale. For the mag- 

 nitude of the star we have G.IG by the mean of the available estimates 

 on record, and G.17 by the observations made at this observatory with 

 the meridian photometer. The resulting magnitude of the satellite is 

 5.28 or 5.29, in close agreement with the value, 5.24, found by a very 

 different method, in the Annals of the Observatory, Vol. XI, p. 276. 



Saturn. — A letter from Prof. Hastings, of the Johns Hopkins Uni- 

 versity, puts an important point in regard to the nature of Saturn-s rings 

 (C, the dusky ring, B, the inner bright ring. A, the outer ring) so briefly 

 that it may be quoted. "The ring C jirojected on Saturn is dark; if 

 the ring were composed of particles having the same albedo as the planet 

 it would appear neither bright nor dark, as it would return to us by re- 

 flection the exact equivalent of what it intercepts. But the albedo of B 

 is greater than that of the planet since it is the brightest part of the 

 system. Hence either the material of C is different from that of B, or 

 the planet is self-luminous." 



Possible planets beyond Neptune. — Prof. Forbes, of Edinburg, and Mr. 



D. P. Todd, of Washington, have, during the year, published accounts 

 of their theoretical in'oofs of the existence of a planet or jjlanets beyond 

 Xeptune. Prof. Forbes founds his conclusions upon the statistical dis- 

 tribution of the aphelia of comets. These have long been known to be 

 grouped at certain distances from the sun, with empty spaces so far as 

 known between groups. There is a group of eleven comets whose aphe- 

 lia are at Jupiter's mean distance, 5, and another group of six comets 

 about Neptune's distance, 30. 



Another well marked group of seven comets have aphelion distances 

 of about 100. 



The theory of the introdution of comets into the solar system, proposed 

 a few years since by Prof. Xewton, of Yale College, is made the basis of 

 the investigation, and according to this, any comet which is drawn into 

 our system from outside and its elements so changed as to make it a 

 permanent member of the system must have been so influenced by a 

 planet which was somewh ere near the aphelion point of this comet's 

 orbit at the time of its introdution. 



If the aphelion points of the seven comets of the last group described 

 be marked on a globe, it is found that they lie on or near a great circle. 

 Prof. Forbes further finds that a planet revolving at a distance 100 in a 

 periodic time of 1,000 years might have been at the points so marked 

 during the period cf two revolutions of the comets of this group, and from 



