Fraught with more difficulty than the detection of the lines alone is the next discovery made upon the disk: the recognition of pairs of lines traversing it.

In 1879, while Schiaparelli was engaged in scrutinizing the strange canali he had discovered on the planet the opposition before, he was suddenly surprised to mark one of them double. Two closely parallel lines confronted him where but a single one had previously stood. So unaccountable did the sight seem, that he hesitated to credit what he saw, being minded to attribute the vision to illusion of some sort and the more so that it was not renewed. While he was still wondering what it meant, the planet parted company with the Earth, carrying its enigma with it.

When the two bodies again drew near to one another in 1882, Schiaparelli set himself to watch for a recurrence of the strange phenomenon. Before long it came, and more bewilderingly than at first; for not one canal alone, but a score of them now showed in duplicate, each presenting to his astonished gaze twin lines perfectly matched and preserving throughout their distance apart. Suspecting diplopia or some other optical trick, he tried various eyepieces to a test of the cause but to no change in the effect. The twin lines continued visible, do what he would, insisting on their own reality in spite of all solicitation to merge. How cautious he was in the matter, and how unwilling at first to believe the evidence of his eyes, is shown by the care he took to guard against deception. It was not until he had assured himself of the reality of the phenomena that he believed what he had seen.

It so chanced that my first experience of the thing was almost equally startling, so unexpected was it and so exceedingly sharp was the definition at the time. It was in an autumn early twilight, through air almost perfectly still, as the light went out of the sky and the markings on the planet began to come forth that the Phison of a sudden showed in duplicate to me, clear-cut upon the disk, its twin lines like the rails of a railway track traversing Aeria. Not more vivid do those of our transcontinental tracks appear as one sees them stretching off into the distance upon our Western plains. More impressive was the sight from the fact that I was not looking for it. It simply suddenly stood forth, this strange parallelism of pencil lines. My surprise matched the wonder of the sight.

Since then I have witnessed it several hundred times, but never with more absolute certainty than at that first fortunate revelation. To this distinctness is due the amazement it then aroused. Not simply because of its surpassing novelty, but for the insistence with which it proclaimed itself was the effect to be ascribed. Less well seen, doubt had robbed it of its full surprise. It requires as a rule steady definition for its initial unmistakable showing, if one would be instantly convinced. Except for such it is not usually easy to the unpracticed, though often discernible to the expert after it has once been seen. But that it is real no one who had had a good view of the sight could doubt; still less after the experience had been repeated over and over again.

What appears to take place is this: where previously a single pencil-like line joined two well-known points upon the disk, twin lines, the one the replica of the other, stand forth in its stead. The two lines of the pair are but a short distance apart, are of the same size, of the same length, and absolutely equidistant throughout their course. It is as if a second line had in some way been mysteriously added to the first since the latter was last seen some weeks before. This in a word is the phenomenon, technically called the gemination of the canals, which has since its discovery called forth so much comment. It is not in reality quite as simple or as sudden as it seems, but this was the way in which the phenomenon was first seen and in which it still continues to be criticised.

Self-assertive of reality, the double lines are patently objective to him who is fortunate enough to see them well. Nevertheless the great difficulty of detecting them, and the still greater difficulty of conceiving how such things can be, has led many not versed in the subject to disbelieve and from that to attempt to explain the sight as illusory. Scepticism seeks self-justification; what is hard of acceptance for its strangeness begetting hypotheses of committed error which find easy credence for their comforting conservatism. Several such have in consequence been propounded to account for the double canals. There is the diplopic theory which credits them to non-focusing; the interferential theory which would make them optical products of the telescope; and the illusion theory which would have them quite simply imaginary.

Inasmuch as in any research the assurance that a phenomenon is real is the first point about it to be established, it is a scientist’s duty, not only to scan the phenomena with jealous care to that end, but to scrutinize every theory which would seek otherwise to account for them—the testing such being only second in importance to observing the things themselves. Accordingly I have examined each of the optical theories that have been advanced and critically compared what they assert and require with the results of observation. The outcome of this research has proved as negativing to any other origin for the double canals than reality as direct observations at the telescope are positive on the point. To show this I shall review each in its consequences, confronting it with what the telescope has to say on the subject; for it is of the pith of the matter that the reality should be as demonstrable on demand as on sight. Furthermore, I shall do this before embarking on the general account of these strange things, because it is vital to any interest that one should be assured from the start of the truth of what he is to read. The preface may seem to him abstruse and prosy, but it will introduce him to some curious optical properties and will eventually enhance his concern by proving to him that what reads like fiction is all the more wonderful for being fact.

I. The Diplopic Theory

Diplopia is the property of seeing double with one eye. Surprising as it sounds it is an effect not unknown to students of optics, though it usually requires training to produce. It is possible only when the eye is not focused on the object, and is not always possible then. From my experiments its feasibility seems to depend upon whether the focus be beyond or before what it should be. If the eye be focused for a point beyond the object, the object is doubled, if for a point this side of it, the latter is simply blurred. When the double is formed, the amount of the separation of the two images is a function of the distance the focus is out. The greater the discrepancy, the wider apart is the ensuing double. Nor does the image, of a line, for example, stop at doubling. After a certain breadth of separation is reached a third line appears, bisecting the interval between the other two. With yet greater widening the third line itself splits into a pair and so the resolution goes on. In my own experiments I have gone so far as to suspect a fifth line. Far from being unconscious, the process of producing the phenomenon is, with some people, of difficult accomplishment. Mr. Lampland, for instance, of the Flagstaff Observatory, to whom we owe the first photographing of the canals, and who sees the doubles of Mars without difficulty, has hitherto found diplopic vision an impossible feat. Even with the most practiced diplopia is never unconscious except when the object viewed, as a micrometer wire, has nothing to locate it in space. Now, the diplopic theory of the double canals supposes that in all cases the eye of the observer is thus unconsciously out of focus.

To this method of their manufacture the telescopic phenomena prove unamenable on five counts.

1. Focusing the eye on an object is now a reflex action, so automatic has it become; in consequence one is commonly directly conscious when an object is not in focus, always so when the object presents detail. Were such not the case we should never, except by chance, see anything defined. Observing through a telescope, after a modicum of practice, differs in no respect from observing in everyday life. Consequently, that an experienced observer should not know his business in so primary a matter is preposterous. One may or may not believe that “the undevout astronomer is mad,” but that the perpetually unfocused one would be is beyond debate.

2. Generically unlikely, the failure to focus is here specifically out of the question. For the observer does not use the canals to focus on for the simple reason that he cannot. Like all delicate detail, the doubles appear not continuously but by flashes of revelation, according as the atmospheric waves permit of passage undisturbed. To focus on them would be next to impossible even were it resorted to—which it never is. By the exponents of the theory this important fact is overlooked: the unforeseen showing of the canals and therefore the absolute lack of complicity of the eye in the matter. What one focuses on is the look of the main markings of the disk. Now, to suppose an observer systematically out in his perceptions of so featureful a planet as that of Mars, so that he does not know when he sees its image sharp, implies a lack of knowledge of astronomic observation in the supposer.

3. Study by the writer shows the width of a given double canal to be constant for a given date. Within the errors of perception or recording the twin lines are always at the same epoch the same distance apart. The greater the number of determinations made, the nearer the result approaches to this mean; and the greater the care used in delineation, the less each value departs from it.

Now, if the thing were a matter of mistaken focusing, an eye could not be thus true to its own mistakes. If it were out in its focus by a certain amount at one time, it would be likely to be out by a different amount at another. So that by the very terms of its making a diplopic double would be sure to vary. Indeed, in laboratory experiments it is impossible to prevent it. For the eye rests itself automatically by change of focus, and if it be not consciously kept awry it reverts as near to the true focus as it can of its own accord.

4. Diplopia might be a respecter of persons, but it certainly could not be one of canals. For a given observer it must be objectively general in its application to the same class of objects. Consequently, if the doubling were diplopic, all canals inclined at the same angle to the vertical—for the tilt might affect the result were the eye astigmatic—should be similarly affected. Parallel canals should parallel each other’s action. With the Martian doubles this is not the case. Of two canals similarly inclined the one will be double, the other not, at the same instant and under conditions that are alike. And this persistently. For gemination is an attribute of certain canals and never of others. At a given season of the Martian year, some canals are regularly double, some invariably single. Night after night and presentation after presentation these idiosyncrasies are preserved: the doubles, always pairs, the single, always alone. Nor does the strength of the line affect the action. The single canals are some of them stronger, some of them weaker, than the doubles seen at the same time.

5. If of diplopic origin the mean width of all the doubles should be the same. For though the diplopic width would vary for a given canal according to the moment, a sufficient number of views would yield a mean width which would be the same for all. Tilt apart, the mean width of one canal would be that of another. Among Martian doubles, on the contrary, I have found the width to be a specific property of the particular canal. Each has its own mean width regardless of inclination, and this individual width differs as between one and another by as much as five to two, or, if we consider such canals as the Nilokeras I and II, by more than ten to two.

Any one of these five points is fatal to the theory; a fortiori all.

II. The Interference Theory

From the wave propagation of light it follows that the image of a bright line made by a lens is not itself a simple bright line but a bright band flanked by alternate dark and bright ones. It has, therefore, been suggested that a bright medial line is here concerned and that the double canal is the first of its dark pair of outriders. But the suggestion does not bear scrutiny.

1. It presupposes a central streak brighter than the rest of the disk to give birth to the twin dark lines. This should itself be visible in the image; but no such bright backbone is seen.

2. It demands a perfectly definite width of separation for a given aperture—which is not that observed.

3. It makes the width a function of the aperture, decreasing as this increases—which is not sustained by observation. Different apertures produce no effect on the widths of the Martian doubles, as the writer has shown (Lowell Observatory Bulletin, No. 5) by a change of aperture from twenty-four to six inches.

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.

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.