The column on the Mars canals did not mention the most interesting and plausible explanation made yet to date for this phenomenon. I forget who suggested it, but it’s out there in the webiverse somewhere. The idea is that Percival Lowell (and perhaps Schiaperelli) were actually seeing the reflections of their own retinas in the eyepieces of their telescopes. So what Lowell actually sketched, presumably, were his own retinal blood vessels.