PLATE XVII. 



CRATER REGION ABOUT THEOPHILUS. PHOTOGRAPHED BY RITCHEY. 

 SCALE, THREE-FOURTHS METER TO MOON'S DIAMETER. 



A portion of this field, including the crater Theophilus, is shown in other plates. This most 

 important structure lies just below the middle of the plate near the margin of the illumination. 



The details of structure of the lunar surface, as shown on the margin of the illuminated field, 

 are better exhibited in this picture than in any other ; perhaps better than in any other photograph 

 that has been published. The more important of them have been noted in the descriptions of 

 preceding plates, but attention may well be called to certain of these features, viz., to the numer- 

 ous shallow craterlets near Theophilus, to sundry wrecked craters in the same field, and to the 

 association of small cones and small craters in the region south (upward on the plate) from 

 Theophilus. 



The frequent deformation of craters by elongation is fairly well indicated by several vulcanoids 

 within the field of view. The invasion of the material of the maria is well shown in the region 

 about Theophilus, and, as before noted, the central peak on the crater floor of that structure with 

 its fairly distinct central pit is admirably depicted. 



It is well to note the passage from the very distinct exhibition of the structures on the termi- 

 nator, the margin of the illuminated field, to the obscurity of similar features when the sun is more 

 than forty-five degrees above the horizon. 



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