Double Canals, first seen by Schiaparelli, [28], [192];

impression of, [193], [204];

two classes of, [224];

require steady definition, [194];

phenomena of, [194], [205], [208], [212], [213];

physical bond between the constituents of, [226];

connection with bays, [232];

optical theories of, [196-203];

not illusions, [195-203], [209];

widths of, [205], [206], [221-224], [229], [230], [233];