 Blink Comparator
General Information
Nothing has yet been found about the design or manufacture of the blink
comparator, which appears homemade and was perhaps built in the 1920s
when proper motion and parallax studies got seriously underway. It is
likely to have been used from the time of manufacture through the
1940's, after which the Brashear Blink
Comparator would have taken its place.
The blink comparator would have been used by McCormick Observatory
astronomers for several purposes. The machine allows an observer to
mount two different photographic plates and then peer at the same sky
position on each of the plates to compare which objects can be seen (or
not seen). If a speck seems to jump or change brightness between the
plates relative to the stationary background stars, then the astronomer
knows he has found something of interest. This could be useful in the
discovery of asteroids, comets, meteors, variable stars, binary stars,
proper motions, and parallaxes. At the time when this comparator was
used, there were astronomers specializing in the study of parallaxes,
proper motions, meteors, and variable stars, so it is obvious that the
machine would have been very useful.
Operation
The blink comparator was built to hold two separate 11 × 8 inch
plates, using several clips on a stage (4) which is at a steep angle to
the horizontal. In order to align the plates so that the microscope
views the same sky position on each of them, the right plate holder is
equipped with a knob to move it horizontally (2) and the left plate
holder is equipped with a knob to move it vertically (1). The carriage
as a whole can be moved in the vertically using the handwheel at the
lower left (3). This would allow the astronomer to slowly move his view
vertically up the photographic plates, comparing different fields of
view to search for objects which changed position between the plates.
Each photographic plate was illuminated from behind by a light source
(5). In front of each plate was an eyepiece (6) which was connected to
a microscope, directed perpendicular to the plates themselves. A small
mirror to the rear of the microscope acted as a switcher so that when it
was oriented in one direction, light from the right hand plate came
through the microscope and when it was oriented the opposite way, light
from the left hand plate came through. On the top of the microscope
apparatus is a knob (7) to manually switch the mirror in this way so
that one could "blink" between two plates. The astronomer had to do the
switching himself, so one can imagine the concentration that it took to
keep a steady blinking rhythm as well as looking out for objects which
had changed in position between the two plates.
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