NEW PLANETS NEAR THE SUN. 39 



Each planet would be seen most favourably when in the part 

 of its path remotest from the earth, so that the planet nearest 

 to the sun would on the whole have the advantage of any 

 difference due to that cause. For, of course, while Mercury, 

 being farther from the sun, approaches the earth nearer when 

 between the earth and sun, he recedes farther from the sun for 

 the same reason when on the part of his path beyond the sun. 



It was perfectly clear that no such planet as Leverrier 

 considered necessary to reconcile theory and observation 

 exists between the sun and Mercury's orbit. It appeared 

 necessary, therefore, to assume that either there must be 

 several smaller planets, or else that a cloud of cosmical dust 

 surrounds the sun. Now it is to be noticed that in either 

 case the entire mass of matter between Mercury and the 

 sun must be greater to produce the observed disturbance 

 than the mass of a single planet travelling at the outside of 

 the region supposed to be occupied either by a group of 

 planets or a cloud of meteorites. 



Leverrier considered the existence of a ring of small 

 planets afforded the most probable explanation. He recom- 

 mended astronomers to search for such bodies. It is note- 

 worthy that it was in reference to this suggestion that M. 

 Faye (following a suggestion of Sir J. Herschel's) proposed 

 that at several observatories, suitably selected, the sun 

 should be photographed several times every day with a 

 powerful telescope. ' I have myself,' he says, ' shown how 

 to give these photographs the value of an astronomical 

 observation by taking two impressions on the same plate 

 after an interval of two minutes. It will be sufficient to 

 superpose the transparent negatives of this size taken at a 

 quarter of an hour's interval, to distinguish immediately 

 the movable projection of a small planet in the middle of 

 the most complex groups of small spots.' 



It was while Leverrier and Faye were discussing this 

 matter, that news came of the recognition of an intra- 

 Mercurial planet by Lescarbault, a doctor residing at 

 Orgeres, in the department of Eure et Loire. The story 



