Another very interesting series of photographs showed the coming and going of the first frost of the season in the antarctic regions of Mars. This frost was first observed and photographed at Flagstaff on the 16th November, 1909, and other photographs were taken on the 22nd of that month. In connection with these, Professor Lowell quaintly remarked that, "To chronicle thus the very weather on our neighbour will convince any one that interplanetary communication has already commenced; and that, too, after the usual conventional manner by mundane greetings." Referring to the photographs, it was pointed out that the human eye can see at least ten times as much as a photograph can show as regards planetary detail. This, though not generally known, is perfectly true, and it may be explained thus: We know that in terrestrial photography the camera will reveal many details which the eye is apt to overlook; and, by very long exposures, even celestial photography will give a similar result. In _planetary_ photography, however, exposures must be very short, and the picture obtained is so very tiny that it cannot show all that the eye could see. Under good conditions, therefore, the eye at the telescope will always see immensely more of the finer details on a planet than any camera could show. The great value of the photographs of Mars lies in the fact that they demonstrate beyond the possibility of doubt the existence of certain fine markings which many observers have seen and drawn, but as to the reality of which others, less skilled or less favourably situated, have been extremely sceptical. If the fine lines had no existence on the planet they could not be photographed. In drawing attention to the details on these photographs Professor Lowell emphatically declared that, "The lines you see are '_certainties_,' not matters admitting of the slightest question, for all their strange regularity. Not only I, but all my assistants, have seen them thousands of times the same, and sometimes with all the clearness and sharpness of etchings or steel engravings. "An optical mistake," he then remarked, "which has latterly been hailed as showing that the lines were not lines but a series of dots, was made the other day in France. The observer saw perfectly correctly, but one with knowledge of the optics of a telescope should have known that the effect observed was the inevitable result of using an aperture which the seeing did not warrant; as he could easily have assured himself by looking at the shattered rings round the synchronous image of a star." It may here be pointed out that these weighty and well-considered declarations--which are a complete answer to M. Antoniadi's bold claim--were made by the most experienced observer of Mars, who, as even his opponents admit, possesses the finest site in the world for his astronomical work, and is equipped with a very perfect instrument.