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ASTRONOMY

the impression that the features in question were the work of the inhabitants of Mars. The studies of these features by other observers have been so numerous that it is impossible even to summarize them. Some have failed to see the channels at all, but the most successful in seeing and drawing them have been the observers at the Lowell Observatory. A careful comparison of the maps of Schiaparelli with each other, and with that of Lowell, shows that there is by no means a complete agreement as to these features. Schiaparelli depicts them as rather obscure and more or less diffused bands or streaks of appreciable breadth, whereas in Lowell’s drawings they appear as sharp, fine, dark lines. The channels seen by Lowell are much more numerous than those seen by Schiaparelli, but at the same time we cannot identify all of Schiaparelli’s channels on Lowell’s map. An additional characteristic of the latter is that the numerous junction points of the channels are frequently marked by round, dark spots, like circular lakes, which do not appear on Schiaparelli’s map. The current interpretation of these features has been contested by Barnard, who seems to have seen the planet under much more favourable conditions than either of the two other observers. He had the advantage of the great aperture of the Lick telescope, and the very fine seeing now and then obtained at Mount Hamilton. In exceptionally favourable circumstances he could see markings so minute, intricate, and abundant that it was impossible to delineate them. They were even more numerous on the so-called seas than on the broad continental regions. They were usually more or less irregular, consisting in delicate differences of shade. Ho straight and sharp lines were seen. Between the black spots which abounded in a certain region were seen diffuse hazy lines, rather irregularly formed (Monthly Notices It. A. S. vol. Ivi. 1896, p. 166). These discrepancies between the two observers are all the more noteworthy, in view of the skill of each, Schiaparelli being known for his conscientious care in avoiding every sort of illusion, and training his eye to see only what is existent, while Barnard is equally conscientious, and had at his disposition telescopic power far exceeding that of Schiaparelli. The apparent discrepancy can, we believe, be reconciled by some considerations on the psychology of vision. If we observe a stippled engraving of such an object as the human face from a certain distance, we see nothing but continuous light and shade. If we seek to draw what we see, we may form the best representation by lines representing the outlines of the human features. But if, instead of studying the engraving at a distance, we look at it through a magnifying glass, we see no lines whatever but only collections of points, perhaps without any welldefined arrangement, thicker in some regions than in others, and of various sizes. We thus learn that the eye possesses a property of representing collections and rows of points to the visual sense as continuous lines of light and shade. From this point of view it is quite natural that such features as those described by Barnard should, when seen with less sharpness, present to the eye the .appearance shown on the maps of Schiaparelli. The discrepancy between the outlines, as drawn not only by the two observers, but by the same observer at various times, shows that there is something shadowy and indefinite, we may even say doubtful, in the objects depicted. Some of the channels are unmistakable. Of others th^ difference of shading is so slight that visual distinctness is scarcely possible, and none of them, perhaps, can be considered as anything but the result of a combination of complex features too minute to be separately visible. In this connexion a short paper by Cerulli in the Astronom.

Nach. (Bd. 146, S. 155) may be read with interest. This observer found that after having observed the channels of Mars for two years through a telescope, he was able, by looking at the moon through an opera-glass, to see similar lines upon its surface, but these seeming lines were shown by observations with the telescope to be unreal, being only the result of the unconscious action of the eye in joining together slight and irregular combinations of light and shade in such a way as to produce regular forms. As this action was the result of a long habit, the fact shows the possibility that long practice in observing may result in an observer seeing objects incorrectly. Could we see Mars with an optical instrument bearing the same ratio to a common telescope that the latter does to an opera-glass, the result might be the same for Mars that Cerulli found for the moon. The doubling of the channels noticed from time to time by some observers must also be placed in the category of doubtful phenomena or probable illusions, and there is on this point an absence of agreement among the observers which must throw doubt on their conclusions. On Schiaparelli’s map and in Lowell’s drawing the double channels are simply parallel ones of equal strength, so far apart that they are distinctly separate objects, though others find the supposed duplicity very difficult to make out. It is conceivable that an observer, under unfavourable conditions, might find these pairs of markings hard to separate; but, were such the case, they would be seen as a very broad band and not as the narrow streaks commonly depicted. The well-known fact that the polar regions of Mars are covered with a white deposit during the Martian winter, of which the greater part melts away in the course of the Martian summer, has given rise to ^ the belief that the planet has an atmosphere yWars> and oceans like our own. Apparent changes in the visibility of different spots at different times seem to confirm this view, by suggesting that portions of the planet are from time to time rendered invisible by clouds. But the telescopic observations of Lowell, and those made with a spectroscope by Campbell, both tend to weaken this view; Lowell, from his own observations, reached the conclusion that the atmosphere must be much rarer than that of the earth and comparatively cloudless, and Campbell, by comparing the spectra of Mars and the moon, could not certainly discern any well-marked difference between them. It is known that the moon has no atmosphere sufficient to produce an absorption spectrum. Hence Campbell’s observations show that if the same thing is not absolutely true of Mars, it must be approximately so, in that no spectral lines are strengthened by the atmosphere to an obvious extent. Just how dense an atmosphere could exist without having any appreciable effect on the lines, especially how much aqueous vapour it might contain, we cannot state with precision, but we seem justified in concluding that the Martian atmosphere is much rarer and much more free from aqueous vapour than that of the earth. The problem of reconciling the supposed winter snow deposits with the great rarity of the atmosphere presents no difficulty. But there is real difficulty in supposing that the summer temperature can be so high as to melt any considerable thickness of snow or ice. The most probable explanation of this phenomenon seems to be that the polar caps are in the nature of hoar frost deposited during the extreme cold that must prevail in a region completely cut off from the rays of the sun, and not warmed as our polar regions are by air currents from the warmer equatorial regions. Ice slowly evaporates at a temperature below the melting-point. It is not, therefore, necessary to suppose that the sun’s rays warm the polar regions of Mars even up to the melting-