208 



THE NEW PLANETS. 



its form. He imagined also that its appearance afforded indications of a diur- 

 nal rotation in twenty-seven hours : this, however, has not been confirmed by 

 subsequent observation. 



The apparent magnitude of Ceres is about six seconds : it is an object of a 

 ruddy color, appears about the size of a star of the eighth or ninth magnitude, and 

 is invisible to the naked eye. It is surrounded with a dense atmosphere, and 

 shows an ill-defined disk. Schroter found, by a great number of observations, 

 that the height of its atmosphere amounted to nearly seven hundred miles that 

 it was very dense near the surface f the planet, and more attenuated at greater 

 heights and that it was subject to changes which produced great variations in 

 the apparent size of the planet. 



Sir William Herschel, about the year 1802, immediately after the discovery 

 of Ceres and Pallas, undertook a series of observations with his powerful re- 

 flecting telescopes, with a view of ascertaining whether either of these planets 

 were attended by satellites. Many minute stars appeared near the disk of 

 Ceres, but none exhibited that change of position which could be supposed to 

 belong to a satellite. His observations fully corroborated those of Schroter. 

 He says that when viewed with a power of 550, Ceres is surrounded with a 

 strong haziness ; the breadth of the coma beyond the disk may amount to the 

 extent of a diameter of the disk, which is not very sharply defined. Were the 

 whole coma and star taken together, they would be at least three times as 

 large as the star. The coma was very dense near the nucleus, but lost itself 

 pretty abruptly on the outside, though a gradual diminution was still very per- 

 ceptible. 



The planet Pallas has a ruddy appearance, but not so much so as Ceres. 

 It is surrounded also by a nebulosity, but not so extensive. The height of its 

 atmosphere, according to Schroter, is about 450 miles, being two thirds of that 

 of Ceres. The light of the planet is eminently subject to those sudden varia- 

 tions which have been taken to indicate irregularity of form. 



Sir William Herschel says, in speaking of Pallas : " I cannot, with the ut- 

 most attention, and under the most favorable circumstances, perceive any sharp 

 termination which might denote a disk ; it is, rather, what I would call a nu- 

 cleus. The appearance of Pallas is cometary, the disk, if it has any, being 

 ill-defined. When I see it to the best advantage, it appears like a much-com- 

 pressed, extremely-small, but ill-defined, planetary nebula. With a twenty- 

 foot reflector, power 477, I see Pallas well. I perceive a very small disk, 

 with a coma of some extent about it, the diameter of which may amount to six 

 or seven times that of the disk alone." These observations were made in 1802. 



Great diversity of opinion has prevailed respecting the actual diameter of 

 the new planets, Herschel estimating all of them to be considerably under 200 < 

 miles, while Schroter maintains that some of them are as large as our moon. \ 

 This diversity is doubtless produced by the extreme smallness of the planets, ' 

 their great distance, and the undefined appearance they have, owing to the ] 

 nebulosity which surrounds them. 



We shall have occasion again to notice the theory which explains them by ] 

 the supposition that they are fragments of a broken planet, when we shall refer < 

 to the subject of meteoric stones. 



