would be on a scale, measured on the Moon's axis, of ten kilometres to the millimetre, or about 160 miles to the inch.
For the convenience of those who may wish to use them, the ends of the polar and equatorial axes are indicated on the maps by short lines, and their intersection at the centre of the disk by a cross. At the time the photograph was taken, the libration in latitude as seen from Jamaica was -5.°63, that in longitude -3°78. The equatorial defect, or breadth of the region not illuminated by the Sun, measured along the equatorial axis, is 0.6 millimetres. Similarly the polar defect is 0.5 millimetres.
Since of all the craters and peaks on the lunar surface which so far have received names but a very small number can be recognised on the Moon itself when it is full, it was evident that a map showing these craters, although founded on a photograph of the full moon, must be constructed largely by hand. In the location of these craters the photographs taken at five different phases proved an indispensable assistance, and by their aid identifications were easily secured which would have been absolutely impossible without them from a mere inspection of the lunar surface with the telescope. It is probable that the total number of craters and craterlets visible upon the Moon under favourable conditions exceeds 200,000 but is less than 1,000,000.
In the key maps of the four quadrants, only those craters are inserted that have received names, while in the iowr photographic maps, besides the craters already named, all the most conspicuous anonymous craters to be found upon the Moon are shown. The conspicuousness of a crater depends primarily on its location, the height of its walls and the brightness of its interior, and only secondarily upon its size. It is probable that the most conspicuous craters not shown upon the map are most of them of small size. At the time of full moon only a small proportion of the named craters are visible, while a large number of the most conspicuous craters are anonymous. Any object upon the Moon not named upon the map may be readily designated with sufficient accuracy for most purposes by its latitude and longitude.
It was first pointed out by Schröter that if we measure the length of the shadow of a lunar peak, and know its distance from the terminator, we can compute the difference of elevation between the peak and the region where its shadow is cast. Many measures were made by Schröter, by Madler and by Schmidt. For some reason the agreement between the results of these different observers is not very satisfactory, the difference averaging about 1,000 feet. A considerable portion of this difference is probably due to
