NEW PLANETS NEAR THE SUN. 55 



strong against the theory that Vulcan was observed. The 

 same reasoning applies to both these bodies. When I 

 speak therefore of planet i, it will be understood that planet 

 2 also is dealt with. First, as this planet appeared with a 

 disc appreciably round, it is clear that it must have been 

 near the point of its orbit farthest from the earth, that is, 

 the point directly beyond the sun. It was then nearly at its 

 brightest. Yet it appeared as a fourth-magnitude star only. 

 We have seen that Lescarbault's Vulcan, even when only 

 half-full, would appear as bright as Mercury at his brightest, 

 if Lescarbault's account can be accepted in all its details. 

 Situated as planet i was, Vulcan would have shown much 

 more brightly than an average first-magnitude star. At 9 

 very moderate computation it would have been twice as bright 

 as such a star. But planet i appeared fainter than a fourth- 

 magnitude star. Assume, however, that in reality it was 

 shining as brightly as an average third-magnitude star. Then 

 it shone with much less than a twentieth of the lustre Vulcan 

 should have had, if Lescarbault's estimate were correct. Its 

 diameter then cannot be greater than a quarter of that which 

 Leverrier assigned to Vulcan on the strength of Lescarbault's 

 observation. In fact, the apparent diameter of planet i, when 

 in transit over the sun's face, could not be more than a six- 

 teenth of Mercury's in transit, or about two-fifths of a 

 second, roughly, about a 5oooth part of the sun's apparent 

 diameter. It is certain that Leccarbault could not have 

 made so considerable a mistake as this. Nay, it is certain, 

 that with the telescope he used he could not have seen a 

 spot of this size at all on the sun's face. 



It will be seen that Lescarbault's observation still 

 remains unconfirmed, or rather, to speak, more correctly, 

 the doubts which have been raised respecting Lescarbault's 

 Vulcan are now more than ever justified. If such a body 

 as he supposed he saw really travels round the sun within 

 the orbit of Mercury, it is certain that the observations 

 made last July by those who were specially engaged in 

 seeking for Vulcan must have been rewarded by a view of 



