Prof. Challis on the Planet within the Orbit of Mercury. 471 
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 33 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 eyeviece 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 
mode of using this apparatus for anguiar measurements will be seen 
by the following account of the observations. The observer had 
also a small transit-instrument by which he obtained true time, using 
for timepiece his watch, which, as it only indicated minutes, required 
the supplement of a temporary seconds’ pendulum. 
In the account which M. Lescarbault gives of his observations, he 
says that it had been his practice to examine with the telescope the 
contour of the sun for a considerable interval on each day in which 
he had leisure, and that at length, on March 26, 1859, he saw a small 
round spot near the limb, which he immediately brought to the inter- 
section of the wires of the telescope. ‘Then, according to his state- 
ment, he quickly turned the graduated card till two of the wires of 
the finder were tangents to the sun’s limbs, or equidistant from them. 
But it is evident that to effect an angular measurement in this way, 
one of the middle wires of the finder must have been placed tangen- 
tially to the sun’s limb at the point of their intersection, to which 
point the spot had just been brought. Assuming that this operation 
was performed, the angular distance of the point from the vertical 
diameter of the sun might be read off, as the account states that it 
was, by applying the plumb-line apparatus to the graduated card. 
This method could only give a rough measure of the angular position 
of a point very near the sun’s limb; and in fact M. Lescarbault does 
not appear to have attempted to determine the position of the spot 
during the interval between the beginning and the end of the 
transit. He states that the spot had entered a little way on the sun 
