ON MAPPING THE SURFACE OF THE MOON. 81 



telescopic view is far superior to the photographic, the continual changes of 

 illuminating and visual angle prevent that appreciation of the relations of 

 different features to certain epochs of production which can be so well studied 

 in the photogram ; the detail thus seized upon by the aid of photography is 

 vividly realized by the eye at the telescope when the surface of the moon is 

 suitably illuminated. 



While engaged upon area IV A"" I have met with a remarkable instance of 

 difference between De La Rue's photogram, Eeb. 22, 1858, and Rutherford's, 

 March 6, 1865. Lohrmann figures a jjlain, bounded on the AV. by a moun- 

 tain-chain on which he gives a little crater, which he numbers 51. In 

 De La Rue's photogram the crater, which appears to be shallow, is exactly in 

 the position given by Lohrmann. Hot a vestige of this crater is to be discerned 

 on RuiherfordJs 2->liotogram seven years later ! Both photograms are under 

 nearly the same illumination. 



I have also met with a few instances of apparent variations of tint and 

 brightness. The floor of the crater Hind, on the S. W. of Hipparchus, appears 

 to have undergone a variability of tint during a period of eleven years 

 according to the following numbers : — 



1858-14=5°-9, 1865-18=7°-0, 1868-98 = 3°-6 



The slopes of two valleys, lY A*" ^ and IV A"" ii, which cut through the 

 S.W. border of Hipparchus, manifest different degrees of brilliancy on the 

 two photograms. They are much brighter on Rutherford's than on De La 

 Rue's photogram, and IV A"* ^ appears to have become brighter in a greater 

 degree than IV A"" i^. 



De La Rue 1858-14 



Rutherford 1865-18 



A crater, IV A^ i^, the middle of three conspicuous craters W. of Hippar- 

 chus, marked E Sec. I. Lohrmann and G by Beer and Miidler, appears to have 

 become brighter since 1858. The gradations are exhibited below : — 



Beer and Madler. . 1831-34 = 7-00 



De La Rue 1858-14 = 6-30 



Rutherford 1865-18 = 7-14 



Birt 1868-98 = 7-56 



Birt 1868-99 = 8-00 



Full moon. 



Terminator just past Copernicus. 

 j> >> jj 



6''80'"past full moon. 



The number of objects on the moon's surface, registered in accordance 



with the plan proposed in 1865 (see Report, 1865, pp. 294-299), is as 

 follows : — 



781 on 135 areas in Quadi-ant Ij 



354 „ 86 „ „ IL 



227 „ 59 „ „ IIL 



737 „ 63 „ „ IV. 



Total 2099 343 „ on the moon's surface. 



Of these, 769 only have been published, viz. 492 in the Reports of this 

 Committee, and 277 in Mr. Birt's Monogram of the ' Mare Serenitatis.' 



1869, 



