farther apart; but in fact diaphragming has no effect on their width. Not only should all the canals appear double, but they should all seem the same width. Less than one eighth of the canals have ever been seen to be double; and the double canals vary from each other in width, ranging from 2° to 5°.
In drawing A: 1 (the Euphrates) appears much wider than 2 (the Astaboras), while 3 (the Protonilus) is narrower than either; 4 (the
Vexillum), which is a double canal, the writer was unable to resolve, but he could never have classed it with 5 (the Astrusapes). This last appeared as a sharp dark pencil mark, as, indeed, do all the single canals, when the seeing is really good. The double canals then come out like the lines of a railway track seen from a half-distant hill.
If the double canals are really due to diffractive effects, how is it that only those are able to distinguish them whose eyesight is sufficiently good to obtain an exceptional view of the planet? Should not any one who can see the single canals be able to see them double?
