PLATE XXIII. 



MARE IMBRIUM AND SURROUNDINGS. PHOTOGRAPHED BY RITCHEY, NOVEMBER 21, 



I9OI, 7 HOURS 32 MINUTES P.M., CENTRAL STANDARD TIME. EXPOSURE, 



ONE SECOND. SCALE ONE-HALF METER TO MOON*S DIAMETER. 



This plate depicts the western two-thirds of the Mare Imbrium : it does not show the interest- 

 ing Sinus Iridum on its northern shore, nor the Harbinger Mountains on its eastern side. The most 

 noteworthy features are the relatively level surface of the mare and the greater vulcanoids and 

 peaks on its margin, or in its midst, and the Alpine valley on its northwest side. 



The great crater near the lower margin of the mare is Plato. This crater has a diameter of sixty 

 miles, and is very nearly circular. It is separated from the M. Imbrium by little more than its own 

 wall, and from the narrow M. Frigoris on the north by a field of upland that declines gently to that 

 mare. This field is thickly beset by small cones. The interior walls of the crater of Plato rise in 

 general to a height of about four thousand feet above its floor. At some points, however, this 

 wall is over seven thousand feet in height. The floor of the crater appears in the plate to be 

 smooth and of a rather even, very dark hue. It is, however, the seat of rather extensive topo- 

 graphical and color features. There are at least six crater cones, about forty patches of peculiar 

 coloration. The failure of these markings and structures to appear on this admirable plate may 

 be taken as a measure of the difference between what is shown by the best reproductions of 

 photographs now obtainable and the revelations of the telescope under the most favorable 

 conditions. 



On the sea south of Plato is a group of remarkable peaks. Those on the extreme right are 

 known as the Straight Range ; those on the center as the Teneriffe Mountains ; the solitary peak 

 yet farther to the west is Pico. 



The wide cleft to the left of Plato, about one hundred miles away, is the Alpine valley. 

 Owing to the high sun it is not well shown. 



The three great vulcanoids near the left-hand margin of the mare are : the largest Archimedes, 

 the intermediate Aristillus, and the smallest Autolycus. 



