An illustration of the conservativeness of astronomers in regard to planetary markings is shown in their cautious attitude concerning the polar snow caps of Mars. Here are white polar caps on Mars, precisely where they ought to be if they _are_ snow, they wax and wane at the time they should and at no other time, a dark band appears at their borders as the caps in turn diminish in size, which has been interpreted as water due to the melting snow, and no other substance known could possibly reproduce these varying conditions. Professor C. A. Young, in describing these white areas, says: "The one which happens to be turned toward the Sun continually diminishes in size, while the other increases, the process being reversed with the seasons of the planet." After these admissions Professor Young cautiously says: "These are believed to be ice caps." Sir John Herschel says: "The variety in the spots may arise from the planet not being destitute of atmosphere and clouds, and what adds greatly to the probability of this is the appearance of brilliant white spots at the poles--one of which appears in our figure--which have been conjectured with some probability to be snow, as they disappear when they have been long exposed to the Sun, and are greatest when just emerging from the long night of the polar winter." Had Michael Faraday been an astronomer, how long would it have taken him to pronounce these white polar caps snow and ice? De la Rive, in his memoir of Faraday, in speaking of his marvellous accomplishments, says: "One may easily understand what must be produced under such circumstances by a life thus wholly consecrated to science, when to a strong and vigorous intellect is joined a most brilliant imagination." Tyndall, in his discourse "On the Scientific use of the Imagination," says: "Bounded and conditioned by co-operant reason, imagination becomes the mightiest instrument of the physical discoverer. Newton's passage from a falling apple to a falling Moon was a leap of the imagination." That Herbert Hall Turner, Professor of Astronomy in the University of Oxford, does not regard the various contributions on the surface features of Mars as belonging to astronomical science may be inferred from his interesting book lately published, entitled "Astronomical Discovery." This book presents to us the history of the discovery of Uranus and Eros, of Neptune, Bradley's aberration of light, Schwabe and sun-spot period, the variation of latitude, etc., but not a word about the marvellous discoveries of the _canali_ of Mars by Schiaparelli, so fully confirmed by the observation and drawings of many others, and the great advances made by Lowell in the discovery of new features with his lucid and rational interpretation of the seeming enigmas.