Page 384 - 89 history of astronomy_opt
P. 384
berg, and the Lick Observatory.
Greenwich Observatory has been here
selected for tracing the progress of accurate
measurement. But there is one instrument
of great value, the heliometer, which is not
used at Greenwich. This serves the purpose
of a double image micrometer, and is made
by dividing the object-glass of a telescope
along a diameter. Each half is mounted so
384
