Light and Colour (rfthe Moon. 55 



usual degrees of the brightness of the seas ; belonging to the 

 darkest of which are the Mare Crisium, some contracted parts 

 of the Mare TranqniUitas, and the Mare Nubium, as well as the 

 margins of the Mare Serenitalis, The seas situated in the more 

 elevated northern latitudes constitute the 3d degree, and conse- 

 quently are distinguished with greater difficulty from the sur- 

 rounding regions. Besides, we here and there find, between 

 bright parts, small, narrow, and tortuous valleys, whose bright- 

 ness is at 2°; and the 2d and 3d degrees of light are also very 

 common at the surface of annular mountains, as well as upon 

 the hilly regions. There are also three high mountains in the 

 region of Pythagoras which stand at 2^° of the scale, whilst all 

 the surrounding regions, including mountain and valley, have a 

 brightness of 4° and 5°. This fact is altogether an anomalous 

 one ; for although there are many hills and mountains in the 

 moon of a sufficiently dark shade, still the whole of the coun- 

 try which surrounds them is likewise of the same tint. 



The brighter regions vary between the 4th and 6th degrees. 

 The last of these two, belongs only to a part of the south-west 

 quadrant of the moon, and to parts near its edges. The mar- 

 gins of most of the aimular mountains range from 4° to T of 

 illumination, and many of them on their external parts present 

 an equal brightness, or very nearly so. The summits of the 

 isolated or concentric mountains have from 6° to 8°, but gene- 

 rally, it is not the most elevated peaks that have this bright- 

 ness. A few, which are not much elevated, attain to the 9th 

 degree. There is, near Atlas, a small system of J;iills, and to 

 the north of Lejcell, a region almost uniform, which reach this 

 degree of brightness ; but, generally speaking, it is in the craters, 

 and in the annular mountains, that we find the last three de- 

 grees of brightness. 



There are some craters which shine brightly not only towards 

 their edges, but also throughout their whole internal surface. 

 These are the clearest of all the almost innumerable luminous 

 points, with which the disk of the moon is studded at the 

 full. A circumstance sufficiently remarkable is this : — that at 

 these times we have no criterion by which to decide whether 

 these points are elevations or depressions. We must wait the 

 period of the phases to determine whether the luminous ap- 



