Photography holds to-day a place of publicity in the exposition of the stars. Directed by Draper to the heavens thirty-four years ago, the camera recorded then the first picture ever taken of the moon. From this initial peering into celestial matters, practice has progressed until now the dry plate constitutes one of the most formidable engines in astronomic research. Not most effectively, however, in the field which might have been predicted. Beautiful as the lunar presentment was, as a presentiment of what was coming, it pointed astray. For it is not in lunar portrayal, superbly as its crater walls in crescent chiaroscuro or its crags that cast their tapering shadows athwart the dial of its plains stand out in the latest photographs of our satellite, that the camera’s greatest service has since been done. Impressive as they are, these pictorial triumphs are chiefly popular, and appeal on their face to layman and scientist alike. Not in the nearest to us of the orbs of heaven, but in the most remote has celestial photography’s most prolific field been found to lie. Its province has proved preËminently the stars, especially the farthest off, and that star-dust, the nebulÆ, from out of which the stars are made. Reason for this explains at once its efficiency and its limitations.

Its rival, of course, is the eye. It is as regards the eye that its comparative merits or demerits stand to be judged. Now, thus viewed, its superiority in one respect is unquestionable; it simply states facts. But though it cannot misinform, it can color its facts by giving undue prominence to the effect of some rays and suppressing the evidence of others, so that its testimony is not, it must be remembered, always in accord with that of human vision. Speaking broadly, however, it is so little complicated a machine as to register its results with more precision than the retina. The evidence of the camera has thus one important advantage over other astronomic documents: it is impersonally trustworthy in what it states. Bias it has none, and its mistakes are few. Imperfections, indeed, affect it, but they are of purely physical occasion and may be eliminated or accounted for as well by another as by the photographer himself.

In trustworthiness, then, so far as it goes, it stands commended; not so much may be said of its ability. This depends upon the work to which it is put. In certain lines it asserts preËminence; in certain others it is so far behind as to be out of the race. The reason for both is one and the same, for, as the French would say: It has the faults of its quality. The very trait that fits it for one function, bars it from the other. This excellence is that by which the tortoise outstripped the hare,—a plodding perseverance. Far less sensitive than the retina the dry plate has one advantage over its rival,—its action is cumulative. The eye sees all it can in the twentieth of a second; after that its perception, instead of increasing, is dulled, and no amount of application will result in adding more. With the dry plate it is the reverse. Time works for, not against it. Within limits, themselves long, light affects it throughout the period it stands exposed and, roughly speaking, in direct ratio to the time elapsed. Thus the camera is able to record stars no human eye has ever caught and to register the structure of nebulÆ the eye tries to resolve in vain.

Where illumination alone is concerned the camera reigns supreme; not so when it comes to a question of definition. Then by its speed and agility the eye steps into its place, for the atmosphere is not the void it could be wished, through which the light-waves shoot at will. Pulsing athwart it are air-waves of condensation and rarefaction that now obstruct, now further, the passage of the ray. By the nimbleness of its action the eye cunningly contrives to catch the good moments among the poor and carry their message to the brain. The dry plate by its slowness is impotent to follow. To register anything, it must take the bad with the better to a complete confusion of detail. For the air-waves throw the image first to one place and then to another, to a blotting of both.

All of which renders the stars, where lighting counts for so much and form for so little, the peculiar province of celestial photography. With the study of the surfaces of the planets the exact contrary is the case. With most of them illumination is already to be had in abundance; definition it is that is desired. What succeeds so excellently with the stars is here put to it to do anything at all. At its best, the camera is hopelessly behind the eye when it comes to the decipherment of planetary detail. To say that the eye is ten times the more perceptive is not to overstep the mark. To try, therefore, here to supplant the eye by the camera is time thrown away.

Of scant importance to the expert in such matters as Mars, there is a side of the subject in which service might be hoped of it: that of elementary exposition. Congenitally incapable of competing with the eye in discovery, the most that, by any possibility, could be looked for would be a recording of the coarser details after the fact. For this reason it had long been a purpose at Flagstaff to photograph some at least of the canals. But the project seemed chimerical. To get an image suitable at all some seconds of exposure would be required, and during such time the shifting air-waves would blur the very detail desired to be got. It was a problem of essential premises mutually annihilative. The more the would-be photographer should avoid the one; the more he would fall into the other.

Nevertheless the thing was tried in 1901. In 1903 the subject was taken up by Mr. Lampland, then new at the observatory. The results were better than those of two years before, the images more clear-cut but still incommunicable of canals. Still they were satisfactory enough to spur to increased endeavor, and during the following interopposition preparations were made to grapple with the planet as successfully as could be devised at its next return. This happened in May, 1905. It then showed a disk only 17'' in diameter, or 1/120 that of the moon,—and this disk Mr. Lampland attacked with the 24-inch and a negative amplificator that increased the focal length of the former to 143 feet. At such focus the planet’s image was received upon the plate. Everything that could conduce to success had been put in requisition. To this end of better definition the color curve of the objective was first got, and for it a special color screen constructed by Wallace. In spite of its name no achromatic is so in fact, but brings rays of different tint to different focus. The color curve shows where these severally lie, and the color screen, a chemically tinted piece of glass, is to absorb all those which would blur the image by having a different focus from the ones retained. Next, all manner of plates were tried. For in these again it was necessary to reconcile two contradictory characters, a rapid plate and a well-defining one. For the coarser the grain the speedier the plate; and coarse grain disfigures the detail. Both qualities on so small an image were obligatory and yet both could not be got. Then the clock had to be as smooth-running as possible. So by a suggestion of Mr. Cogshall’s one was obtained that filled this requisite, a new form of conical pendulum. Upon this a further refinement was practiced. Ordinarily clockwork is timed to follow the stars; this was altered to follow the planet, and so keep it more nearly motionless while its picture was being taken. Then the device of capping down the telescope to suit the air-waves, which had been found so effective to the bringing out of fine detail, was put in practice. Lastly, all developers were tried, and those found suited to the finest work were used.

Many pictures were taken on each plate one after the other, both to vary the exposure and to catch such good moments as might chance. Seven hundred images were thus got in all; the days of best definition alone being utilized. The eagerness with which the first plate was scanned as it emerged from its last bath may be imagined, and the joy when on it some of the canals could certainly be seen. There were the old configurations of patches, the light areas and the dark, just as they looked through the telescope, and never till then otherwise seen of human eye, and there more marvelous yet were the grosser of those lines that had so piqued human curiosity, the canals of Mars. He who ran might now read, so that he had some acquaintance with photography. By Mr. Lampland’s thought, assiduity, and skill, the seemingly impossible had been done.

After the initial success was thus assured, plates were taken at other points around the planet and other well-known features came out; “continents” and “seas,” “canals” and “oases,” the curious geography of the planet printed for the first time by itself in black and white. By chance on one of the plates a temporal event was found registered too, the first snowfall of the season, the beginning of the new polar cap, seen visually just before the plate happened to be put in and reproduced by it unmistakably. Upon the many images thirty-eight canals were counted in all, and one of them, the Nilokeras, double. Thus did the canals at last speak for their own reality themselves.