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as to slide a distance of several inches each
way on an arc whose centre is the focus.
The amount of the movement can be accu-
rately read. Thus two fields of view over-
lap, and the adjustment is made to bring
an image of one star over that of another
star, and then to do the same by a displace-
ment in the opposite direction. The total
movement of the half-object glass is dou-
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