A canal, then, alters in visibility for some reason connected with itself. It grows into recognition from intrinsic cause. But, during all its metamorphoses, in one thing, and in one thing only, it remains fixed,-- in position. Temporary in appearance, the canals are apparently permanent in place. Not only do they not change in position during one opposition; they seem not to do so from one opposition to another. The canals I have observed this year agree fairly within the errors of observation with those figured on Schiaparelli's chart.

The fact that in all cases they do not absolutely agree with his is the very best of proofs that they are substantially the same; for such slight discordance proves the absence of conscious psychic reproduction. It confirms by not conforming.

As, in observations of minute detail, the psychic element insensibly creeps in, it will be well to consider it for a moment. An idea is a force, a mode of motion, which, unless obstructed by other ideas, instantly and inevitably produces its effect upon whatever mind it may chance to impinge, just as light or electricity or any other mode of motion does, according to its kind. An easy instance of this can be got by asserting at dinner, before a company of connoisseurs, that the wine is slightly corked. Every one not actuated by a spirit of contradiction will at once perceive that it is so, and will continue to believe it, in many cases, after it is abundantly disproved. This is what takes place in the normal, unbiased--that is, so far as this idea goes-- vacant mind. But minds have their familiar ideas, which an incoming idea is pretty sure to rouse, and these react to some extent upon the stranger, and color it with something of their own complexion. If we expect to meet a certain person, an approaching figure will most deceitfully take on his garb. The mere idea of a man walking finds the expectation ready instinctively to endow it with the attributes of our friend. But this may happen truly as well as falsely. The expert sees what the tyro misses, not from better eyesight but from better mechanism in the higher centres. A very slight hint from the eye goes a long way in the brain of the one; no distance at all in the brain of the other.

Our senses are our avenues of approach from the outer world. Messages from them are therefore usually and rightly attributed to stimuli from without. But it is possible for these messages to be tampered with at any stage of their journey. It is even possible for them to be started in some other part of the brain, travel down to the lower centres and be sent up from them to the higher ones, indistinguishable from bona fide messages from without. Bright points in the sky or a blow on the head will equally cause one to see stars. In the first case the eyes were duly affected from without; in the second, the nerves were tapped to the same effect in mid-route; but in each case the subsequent current travels to the higher centres apparently as authentic the one as the other.

Hallucinations of one sort and another occur in this way. More common, however, are unconscious changes in an originally quite veridic message. We easily see what we expect to see, but with great difficulty what we do not. This may be due to individual idiosyncrasy, or it may be due to a prevailing idea of the time, affecting people generally, in which we unwittingly share. Fashion is as potent here as elsewhere. The very same cause will show us at one time what we remain callously blind to at another. A few years ago it was the fashion not to see the canals of Mars, and nobody except Schiaparelli did. Now the fashion has begun to set the other way, and we are beginning to have presented suspiciously accurate fac-similes of Schiaparelli's observations.

In any observation, the observer is likely to be unconsciously affected in some way or other pro or con, which, from the fact that he is unconscious of it, he is unable to find out. The only sure test, therefore, is the seeing what no one else has seen, the discovery of new detail. Next to that is not too close an agreement with others. Inevitable errors of observation, to say nothing of times and seasons, distance and tilt, are certain to produce differences, of which one has ample proof in comparing his own drawings with one another. Even too close agreement with one's self is suspicious. In the matter of fine detail, absolute agreement is therefore neither to be expected nor to be desired.

All the changes so far observed on the planet's disk are, I believe, capable of explanation either by errors of observation or by seasonal change. For, as is the case with the Earth, not only must vegetation produce