56 ROUGH WAYS MADE SMOOTH. 



that planet. In July, Lescarbault's Vulcan could not have 

 been invisible, no matter in what part of his orbit it might 

 be, and the chances would have been greatly in favour 

 of its appearing as a very bright star, without telescopic aid. 

 But on the other hand it seems extremely probable, in 

 fact, unless any one be disposed to question the veracity of 

 the observers, it is certain, that within the orbit of 

 Mercury there are several small planets, of which certainly 

 two, and probably three, were seen during the eclipse of 

 July 29, 1878. All these bodies must be beyond the range of 

 any except the most powerful telescopes, whether sought 

 for as bright bodies outside the sun (not eclipsed) or as dark 

 bodies in transit across the sun's face. The search for such 

 bodies in transit would in fact be hopeless with any 

 telescope which would not easily separate double stars one 

 second of arc apart. It is with large telescopes, then, and 

 under favourable conditions of atmosphere, locality, and 

 so forth, that the search for intra-Mercurial planets in 

 transit must in future be conducted. As the observed 

 disturbance of Mercury's perihelion, and the absence of 

 any corresponding disturbance of his nodes (the points 

 where he crosses the plane of the earth's motion) show that 

 the disturbing bodies must form a ring or disc whose 

 central plane must nearly coincide with the plane of 

 Mercury's path, the most favourable time for seeing these 

 bodies in transit would be the first fortnights in May and 

 November ; .tor the earth crosses the plane of Mercury's 

 orbit on or about May 8 and November 10. I believe that 

 a search carried out in April, May, and June, and in 

 October, November, and December, with the express 

 object of discovering very small planets in transit, could 

 not fail to be quickly rewarded, unless the observations 

 made by Watson and Swift are to be wholly rejected. 



[Since this was written, Professor Swift has expressed the opinion 

 that his planet cannot possibly have been the one seen near Theta 

 Cancri by Professor Watson, who it seems saw Theta in the centre of 

 a large field of view, and must therefore have seen Swift's planet had 



