54 Light and Colour of the Moon. 



only in the lowest portion of the surface of the ground. Nor 

 is the formation of terraces common, except in cavities in the 

 interior of the great mountains. Our satellite thus exhibits its 

 mountains in their original form, an occurrence which is seldom 

 or never witnessed on the earth. 



7. Light and Colour of the Moon. — That we may complete 

 this general description of the moon's surface, it is necessary 

 to point out the variety of colouring which may be detected. 

 The difference betweeen the bright and the dark parts may 

 be detected by the naked eye, particularly at full moon : and 

 this circumstance proves, that, so far as these appearances 

 are concerned, shadows are not at all the cause, and that 

 differences in the level have no share in these differences of 

 shade. All the shadows which shew themselves on the moon 

 at the epoch of her phases, are always perfectly black, whilst 

 the darker portions of her surface, such as Grimaldi, Plato, 

 Boscovich, and the small spots in Petavius, William Hum- 

 boldt, and Alphonso, are always grey only ; so that the finest 

 points of shadow which are projected over these surfaces can 

 be measured as accurately as in the brighter districts. In 

 the moon, therefore, there are differences as to the quantity of 

 the solar light which is reflected : and as we see very distinct 

 traces of such a diversity in the obscure portion of the moon, 

 ihis same phenomenon occurs likewise in the earth's light, and 

 in all other kinds of light. 



It is proper, therefore, that a scale of these gradations of light 

 should be established, and such an one as the eye, assisted with 

 good instruments, may be able to distinguish its divisions with 

 some degreeof certainty. Preceding selenographers were content 

 with three degrees. Schroeter, and after him M. Lorhmann, 

 have adopted a scale of ten degrees, and we, in this respect, 

 have followed their example. The zero of the scale corres- 

 ponds to the shadow which is projected by the mountains. 

 The first three degrees may be denominated grey, the fourth and 

 fifth, light grey, the sixth and seventh white, and the last three 

 shining white. The 1st, 9th, and 10th degrees are found only 

 on small parts of some spots. The first is that of Grimaldi 

 and Riccioli ; Boscovich, Julius Caisar, Plato, and a part of 

 Schikard have 1^ degrees of light. Numbers S and 3 are the 



