4. Under like optical conditions the optically produced doubles would be all of a width; while the Martian ones show idiosyncratic widths, each peculiar to itself. _III. The Illusion Theory_ Known also as the Small Boy Theory from the ingenuous simplicity on which it rests, this theory attacks the reality of the doubles by questioning that of the canals _en bloc_. Because some boys from the Greenwich (Reform or) Charity School, set to copy a canal-expurgated picture of the planet, themselves supplied the lines which had preceptorily been left out, the Martian canals have been denied existence; which is like saying that because a man may see stars without scanning the heavens, therefore those in the sky do not exist. As to the instructions the boys received we are left in the dark. It looks as if some leading questions had unconsciously been put to them. At all events, English charity boys would seem to be particularly pliant to such imagination, for when Flammarion retried the experiment with French schoolboys, and even inserted spaced dots for the canals in the copy, not a boy of them drew an illusory line. The fact is, this is one of those deceptive half-truths which is so much more deleterious than an unmitigated mistake. Under certain circumstances it is quite possible to perceive illusory lines, due either to shadings otherwise unmarked and thus synthesized or to immediately precedent retinal impressions transferred to places where they do not belong by rapid motion of the eye, as I had myself discovered before the English experiment had been tried. But, as I have also found out, these effects are produced only at the limit of vision, and in that limbo of uncertainty the whole art of the observer consists in learning to distinguish the true from the false. Strength of impression, renewed effect _in situ_, and a peculiar sense of reality or the reverse enable him to adjudge the two. More experience than the boys possessed would have helped them to part the sheep from the goats. But, furthermore, and fatally to the theory here in question, the Martian canals when well seen are not at the limit of vision as its framers supposed, but well within that boundary of doubt; so that the premise upon which the whole theory rests gives way. Under good atmospheric conditions the canals are comparable for conspicuousness to many of the well-recognized Fraunhofer lines and are just as certainly there. Thus each attempt to prove the doubles non-objective turns out when specifically examined to be inconsistent with the facts. With the assurance of their reality thus made doubly sure, we pass to consideration of the things themselves. CHAPTER XVIII THE DOUBLE CANALS I