Page:The Moon (Pickering).djvu/140

98 (d) Every region is shown at five different phases, many details being conspicuous at one phase of the Moon that are not seen at all at another. In this way changes in the snow patches and in the vegetation are shown which could not possibly be indicated by any single photograph.

(e) Sometimes the same region contains some very bright and some very dark areas, such as a bright mountain mass and a dark mare. In such cases, both cannot be shown to advantage on the same photograph. By having five photographs, however, the exposures and printing can be so adjusted that every object will be shown with a suitable exposure upon at least one plate.

(f) All the plates are on the same scale, 5″ to the millimetre, and while there are some differences in the linear dimensions which are unvoidable by this plan on account of the varying libration and distance, there is no great range of scale such as occurs in some photographic atlases, where one plate may be on more than twice the scale of another.

(g) All the plates are approximately oriented parallel to the lunar axis, with south at the top.

(h) Since all the plates are printed the same size as the original negatives, there has been no enlargement of the grain of the plate, and we have in consequence a much smoother surface than is possible in the case of those pictures enlarged from negatives taken with a lens of shorter focus.

(i) A considerable overlapping of the plates is allowed. This is a great convenience when working near the edge of a region.

(j) The photographs were taken, whenever possible, under favourable libration, and especial attention was paid to this point in the original plan.

(k) No shading of the limb was permitted, therefore every region appears in its true photographic relations of light and shade.

In the following pages an alphabetical index is given of all the formations upon the Moon, together with the quadrants in which they are situated and their approximate longitudes and latitudes. By this means they can be readily found upon the maps. The northwestern quadrant is designated as i, the northeastern as 2, the southeastern as 3 and the southwestern as 4.