NEW PLANETS NEAR THE SUN. 43 



Vulcan at the same distance about two-thirteenths of 

 Mercury's. But Vulcan, being nearer the sun than Mercury 

 in the ratio of 147 to 387, or say 15 to 30, would be more 

 brightly illuminated in the ratio of 39 times 39 to 15 times 

 15, or nearly as 20 to 3. Hence if we first diminish Mercury's 

 lustre when at his greatest apparent distance from the sun in 

 the ratio of 2 to 13, and increase the result in the ratio of 20 

 to 3, we get Vulcan's lustre when he is at his greatest apparent 

 distance from the sun. The result is that his lustre should 

 exceed Mercury's in the same degree that 40 exceeds 39. Or 

 practically, for all the numbers used have been mere approxi- 

 mations, the inference is that Vulcan and Mercury, if both 

 seen when at their greatest distance from the sun during 

 eclipse, would probably shine with equal lustre. But in 

 that case Vulcan would be a very conspicuous object indeed, 

 at such a time ; for Mercury when at his greatest distance 

 from the sun, or greatest elongation, is a bright star even on 

 a strongly illuminated twilight sky ; moreover, Vulcan, when 

 at either of his greatest elongations, ought to be visible in 

 full daylight in a suitably adjusted telescope. For Mercury 

 is well seen when similarly placed, and even when much 

 nearer to the sun and on the nearer part of his path where 

 he turns much more of his darkened than of his illuminated 

 hemisphere towards us. Venus has been seen when so near 

 the sun that the illuminated portion of her disc is a mere 

 thread-like sickle of light. Nay, Professor Lyman, of Yale 

 College, in America, has seen her when so near the sun that 

 she appeared to be a mere circular thread of light, the 

 completion of the circle being the best possible proof how 

 exceedingly fine the thread must have been, and also how- 

 small its intrinsic lustre. 



This is indeed the chief difficulty in Lescarbault's 

 supposed observation. If he really saw a body in transit 

 across the sun, moving at the observed rate, and having 

 anything like the observed diameter, that body ought to have 

 been seen repeatedly during total eclipses of the sun, and 

 ought not to have escaped the search which has been made 



