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THE POPULAR SCIENCE MONTHLY.
effect of such difference. When the limit of separating power for a telescope of a particular aperture is set at 1″ or 2″, as the case may be, it is assumed that the stars composing the doubles on
Map No. 21.
which the test is made shall be of nearly the same magnitude, or at least that they shall not differ by more than one or two magnitudes at the most. The stray light surrounding a comparatively bright star tends to conceal a faint companion, although the tele-
