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TOTAL ECLIPSE OF THE SUN, JULY 28, 185]. 337 
seen on the moon’s limb at the eclipse of 1842, it seemed very desirable to have 
some means of noting with accuracy the positions of any objects of a similar nature 
that might appear at the approaching eclipse; and in a letter in the Athenaeum 
of 12th July 1851, I suggested a species of position micrometer suitable for that 
purpose. The instrument there described, with a slight addition, was constructed 
for me by Mr Joun Ante, and its performance proved very satisfactory. It consists 
of a circular plate of metal, AB, fig. 1, (Plate XI.) 8 inches in diameter, attached 
to the sliding tube of the telescope by a split collar with a tightening screw, 
not seen in the figure, so as to prevent it from turning round. The face of this 
plate, next the eye-end of the telescope, was covered with a disc of card, attached to 
it by four screws, eeee. Inside the tube carrying the plate, another tube carrying 
the eye-piece slid smoothly, so as to admit of being freely turned round. To this 
were attached, by another split collar and clamping screw, two springy arms, FC, 
FD, bearing steel points, by which holes could be pricked in the card disc, and a 
small level, G, was fixed at right angles to one of the arms. In the eye-piece were 
three equidistant parallel spider-lines, ab, ed, ef, fig. 2; the two outer, ab, cd, being 
placed at an interval equal to the apparent diameter of the moon, calcu- 
lated for the time of the total phase of the eclipse; so that when they were made 
to embrace the moon’s disc, gh, the middle wire would pass through its centre, o. 
The instrument was adjusted for observation by making the middle wire coincide 
with a plumb line, seen at a distance of about 150 yards, while at the same time 
the bubble of the level was brought to the middle of its tube; and the arms with 
the level were then clamped to the tube carrying the eye-piece. 
When this adjustment was completed, it is obvious that the wires in the eye- 
piece would point vertically whenever the bubble of the level was again brought to 
the middle of the tube. If now the bubble were brought to the middle of the 
tube, while the outer wires were made to embrace the moon's disc, the middle 
wire would pass through its vertex, yg; and two holes being pricked in the card, 
the line joining them would represent the moon’s (or, with sufficient accuracy, 
the sun’s) vertical diameter at the moment of observation. If next, while the 
moon was still kept between the outer wires, the middle wire were made to bisect 
any object, h, near its limb, the wires now having the positions a’)’, cd’, ¢/’, and 
holes were again made in the card, the angle between the lines joining the 
respective pairs of holes would measure, goh, the angular distance of the object 
from the sun’s vertex. It is easy to see how, in this manner, the positions of 
the red prominences seen during a total eclipse, could be rapidly registered on 
the card without ever removing the eye from the telescope. In order to repeat 
the observations, the steel points admitted of being moved in longitudinal slits 
in the arms. so as to describe circles of different radii on the card; and the 
reading point was distinguished from the other by being placed a little farther 
from the centre. 

