Mars is a very difficult object to observe, the atmosphere surrounding it being sometimes so cloudy and foggy that the sight can hardly penetrate through its vapors. When this planet is observed under favorable atmospheric conditions, and with sufficient magnifying power, its surface, which is of a general reddish tint, is found to be diversified by white, gray and dark markings. The dark markings, which are the most conspicuous, almost completely surround the planet. They are of different forms and sizes, and very irregular, as can be seen on Plate VIII., which represents one of the hemispheres of this planet. Many of them, especially those situated in the tropical regions of the planet, form long narrow bands, whose direction is in the main parallel to the Martial equator. The dusky spots differ very much, both from one another and in their several parts, as regards intensity of shade. Some appear almost black, while others which appear grayish, are so faint, that they can seldom be seen. In the southern hemisphere, the darkest part of the spots is generally found along their northern border; especially where there are deep indentations. Some observers have described these spots as being greenish or bluish, but I have never been able to see the faintest trace of these colors in them, except when they were observed close to the limb, and involved in the greenish tinted ring which is always to be seen there. It is probably an effect of contrast, since green and red are complementary colors, and since this greenish tinge around the limb covers all kinds of spots, whether white or dark. When such dark spots, involved in the greenish tint, are carried by the rotation towards the centre of the disk, they no longer show this greenish color. To me, these spots have always appeared dark, and of such tints as would result from a mixture of white and black in different proportions; except that on their lighter portions they show some of the prevalent reddish tint of the Martial surface. It is to be remarked that in moments of superior definition of the telescopic image, the intensity of darkness of all the spots is considerably increased--some of them appearing almost perfectly black. The markings on the surface of Mars are now tolerably well known--especially those of its southern hemisphere, which, owing to the greater proximity of the planet to our globe when this hemisphere is inclined towards the earth, have been better studied. Those of the northern hemisphere are not so well known, since when this hemisphere is inclined towards us, the distance of Mars from the Earth is 26,000,000 miles greater, so that the occasions for observing them are not so favorable.