Oh, how they hated this thing.  You put your eye (just one) against the brass ocular, and stared and stared and stared, while images jumped before your eyes.  It made a clack-clack-clack noise, and gave off a smell of burned dust.  While interviewing Henry Giclas (who was on staff at Lowell Observatory) I asked exactly how the comparator was positioned in its room on the first floor of the Main Building; they had arranged it near the window, so you could rest your eyes looking outside.  You needed to take a lot of breaks.

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