219 



2 " Some account of the Geological Discoveries in the Arctic 

 Regions." 



March 12, 1860. 



The Rev. Professor Challis made a communication " On the Planet 

 within the orbit of Mercury, discovered by M. Lescarbault." 



By a recent comparison of the theory of Mercury's orbit with 

 observation, M. Leverrier found that the calculated secular motion 

 of the perihelion of that planet requires to be increased by 38", and 

 that this difference between observation and theory cannot be ac- 

 counted for by the attractions of known bodies of the solar system. 

 In a letter addressed to M. Faye, and published in the Paris Meteo- 

 rological Bulletins of October 4, 5, and 6, 1859, he suggested that 

 the difference might be due to the attraction of a group of small 

 planets circulating between Mercury and the Sun. On December 22 

 of the same year, M. Lescarbault, a physician and amateur astro- 

 nomer, residing at Orgeres, about sixty miles south-west of Paris, 

 announced in a letter to M. Leverrier that he had seen on March 26, 

 1859, a small round spot traversing the sun's disk, which he con- 

 sidered to be a planet inferior to Mercury. Naturally much inter- 

 ested by this information, M. Leverrier went to Orgeres on Decem- 

 ber 31, and after closely interrogating M. Lescarbault respecting the 

 particulars of the observation, and the instrumental means by which 

 it was made, he returned with the conviction that the observation 

 was trustworthy, and that a new planet had been discovered 

 (Comptes Rendus, January 2, 1860, p. 40). 



M. Lescarbault had long conceived the idea of detecting inferior 

 planets by watching the sun's disk for transits, and in 1858 he put 

 his project into execution. He was in possession of a good telescope 

 of 3f inches aperture and 5 feet focal length, mounted with an alti- 

 tude and azimuth movement, and provided with a finder magnifying 

 6 times. The power of the eyepiece employed in the observations 

 of March 26 was 150. Not being furnished with a position- circle, 

 he adopted the following means of obtaining angular measurements. 

 The eyepiece of the telescope and the eyepiece of the finder each 

 had at its focus two wires crossing at right angles, and the wires of 

 the latter were so adjusted that a star seen at their intersection was 

 seen at the same time at the intersection of the wires of the telescope. 

 There were also in the eyepiece of the finder two wires parallel to, 

 and on opposite sides of, each cross-wire, and distant by about 16'. 

 A circular card about 6 inches in diameter, and graduated to half 

 degrees, was placed concentric with the tube of the eyepiece of the 

 finder, and apparently could be moved both about the tube and, with 

 the tube, about the axis of the finder. A cross-wire of the telescope 

 and a cross-wire of the finder were adjusted vertically by looking at 

 a distant plumb-line, and the diameter of the card containing the 

 zero of its graduation was placed vertically by means of a small plumb- 

 line and eye-hole approximately arranged for that purpose. The 



