NEW PLANETS NEAR THE SUN. 37 



detect the difference of aspect between the disc of Neptune 

 and the point-like image of a star (the feature by which 

 Galle, it will be remembered, recognised Neptune), a single 

 night would have sufficed for the search over the smaller of 

 the above-mentioned regions, and two nights for the 

 search over the larger. The search over the smaller, as 

 already stated, would have revealed the disturbing planet 



On the other hand, the astronomer could not determine 

 the direction of an intra-Mercurial planet within a con- 

 siderably larger space on the heavens, while the search over 

 the space within which such a planet was to be looked for 

 was attended by far more serious difficulties than the search 

 for Neptune. In fact, it seems as though, even when 

 astronomers have learned where to look for such a planet, 

 they cannot expect to see it under ordinary atmospheric 

 conditions when the sun is not eclipsed. 



Let us consider the history of the search for an intra- 

 Mercurial planet from the time when first the idea was 

 suggested that such a planet exists until the time of its 

 actual discovery for so it seems we must regard the obser- 

 vations made during the total eclipse of July, -1878. 



On January 2, 1860, M. Leverrier announced, in a 

 paper addressed to the Academy of Sciences, that the 

 observations of Mercury could not be reconciled with the 

 received elements of the planet. According to those 

 elements, the point of Mercury's orbit which lies nearest to 

 the sun undergoes a certain motion which would carry it 

 entirely round in about 230,000 years. But to account for 

 the observed motions of Mercury as determined from twenty- 

 one transits over the sun between the years 1697 and 1848, 

 a slight increase in this motion of the perihelion was 

 required, an increase, in fact, from 581 seconds of arc in a 

 century to nearly 585. The result would involve, he showed, 

 an increase in our estimate of the mass of Venus by a full 

 tenth. But such a change would necessarily lead to 

 difficulties in other directions ; for the mass of Venus had 

 been determined from observations of changes in the 



