When italics were used in the original book, the corresponding text has been surrounded by _underscores_. The oe ligature has been replaced by the letters oe. The Greek letter lambda has been represented as [lambda]. Some presumed printer's errors have been corrected. These have been listed in a second transcriber's note at the end of the text. DIRECTOR OF THE OBSERVATORY AT FLAGSTAFF, ARIZONA; NON-RESIDENT PROFESSOR OF ASTRONOMY AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY; FELLOW OF THE AMERICAN ACADEMY OF ARTS AND SCIENCES; MEMBRE DE LA SOCIÉTÉ ASTRONOMIQUE DE FRANCE; MEMBER OF THE ASTRONOMICAL AND ASTROPHYSICAL SOCIETY OF AMERICA; MITGLIED DER AST…
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