214 EEMARKS ON THE SMALL PLANETS 



IX. 



ASPECT, PHYSICAL CONSTITUTION, AND REAL MAGNITUDE OF THE ASTEROIDS. 



At the commencement of the present century Schroeter devoted 

 himself particularly to the telescopic study of the four planets Ceres, 

 Pallas, Juno, and Vesta. The care which this astronomer employed 

 in his observations gave great importance to the results of his labors. 

 Hence these results were long admitted without dispute, and we find 

 them reproduced in the most recent French treatises, although, for 

 the most part, they have been shown some years ago to be inexact. 

 Thus Schroeter had deduced from multiplied measurements the ap- 

 parent diameters of the four planets, and had concluded that the 

 largest amongst them. Pallas, presented nearly the same magnitude 

 as the moon. William Herschel, on the other hand, obtained num- 

 bers considerably lower, and yet these numbers were still too high, 

 by reason of the effects of irradiation. This we ascertain from the 

 observations made, under unusually favorable circumstances, by 

 MM. Majdlor and Lamont, with the aid of the excellent refractors 

 of Dorpat and Munich. From the opposition of 1836. M. Lamont 

 found 0".51 for the angular diameter of Pallas, at the mean distance 

 2.77, which gives about 250 leagues for the real diameter of the 

 planet. In April and May, 1847, M. Masdler found in like manner 

 one hundred leagues for the diameter of Testa. It is equally to the 

 effects of the irradiation that we must refer the cause of the vapor- 

 ous appearances assumed by Ceres and Pallas in the telescope of 

 Schroeter; appearances so marked that the astronomer of Lilienthai 

 attributed to those planets atmospheres 200 leagues in height. The 

 great refractor of Dorpat exhibits the disc of Pallas as clear as that 

 of Vesta, and thus vanishes one of the analogies which has been often 

 insisted upon between the comets and the asteroids. 



The existence of a very considerable and turbulent atmosphere 

 would have sufficed, according to Schrceter, to explain the remark- 

 able variations which that astronomer had detected from day, to 

 day in the brilliancy of Pallas or Ceres. At present it is neces- 

 sary to seek elsewhere the cause of this phenomenon, the reality of 

 which has in the mean time been confirmed by later observations. 

 It is probable that the asteroids, as an effect of their daily movement, 

 present to us in succession, regions unequally adapted to reflect the 

 light of the sun. But differences of this nature do not, according to 

 M. Littrow, sufficiently account for the rapidity and extent of the 

 variations observed, and, above all, for the truly stellar brilliancy with 

 which the planet Vesta sometimes sparkles, even when its disc pre- 

 sents no appreciable dimensions. The astronomer of Vienna thinks 

 that the asteroids are of irregular or polyhedric forms, and that they 

 sometimes turn towards us lustrous facets like those of the diamond, 

 or even endowed perhaps with an intrinsic light. However this may 

 be, these variations of brightness are observed in several of the 



