PLATE XVI. 



COPERNICUS AND KEPLER. PHOTOGRAPHED BY RITCHEY. SCALE, ONE-HALF METER 



TO moon's DIAMETER. 



The following ten plates were photographed by G. W. Ritchey with the forty-inch Yerkes 

 refractor, with color screen and isochromatic plate. As will be noted, they in part repeat the 

 features exhibited by the other plates of this series, yet in all instances they serve to supplement 

 or extend the information afforded by them. 



The most important features exhibited by plate XVI are the systems of bright rays of Coperni- 

 cus, Kepler, and Aristarchus. These three ray systems, though less extensive than those of Tycho, 

 taken together constitute the greatest exhibition of the bright bands that exist over the northern 

 part of the surface. The complex branched nature of these bands is particularly well shown, bet- 

 ter, indeed, than the writer has ever been able to note with the telescope. The fact that the 

 bright bands of each system are prolongations of a central bright field is tolerably well shown. 



Although owing to the high sun and the consequent absence of shadows, Copernicus in this 

 view hardly appears as an elevation, it is, under favorable conditions of illumination, perhaps the 

 noblest object on the moon. The wall on the eastern side, according to the estimates of Schmidt, 

 rises to a height of twelve thousand feet above the adjacent plain. The outer slopes of the cone 

 are strongly ridged as by the flow from the crater of lavas which cooled on the steep slopes ; some 

 of these are faintly traceable in the plate. 



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