1868.] 



the Surface of the Muon, 



233 



Strictly speaking, there should be at least three drawings of a ring- 

 mountain — in morning light, at midday, and in evening. It would be 

 better to have five drawings, one at sunrise and another at sunset being 

 added to the three already named. It will be found most convenient 

 to make the drawings within two hours of the moon's meridian passage. 



Shadows thrown from objects on the moon have exactly the same 

 character as those observable on the earth. They are all margined by 

 the penumbra due to the sun's diametral aspect ; this is always traceable ex- 

 cept very near to the object ; but in consequence of the smaller dia- 

 meter and more rapid curvature of the moon's surface, the penumbral 

 space is narrower. At the boundary of light and shade, on a broad 

 grey level tract, the penumbral space is about nine miles broad, quite 

 undefined, of course, but perfectly sensible in the general eifect, and worthy 

 of special attention while endeavouring to trace the minute ridges (of 

 gravel?) or smooth banks (of sand?), which make some of these surfaces 

 resemble the postglacial plains of North Germany, or central Ireland, or 

 the southern parts of the United States, which within a thousand centuries 

 may have been deserted by the sea. 



To the same cause is due the curious and transitory extension of half- 

 lights over some portions of the interior of craters, while other neighbour- 

 ing portions have the full light. The effect is occasionally to produce 

 half-tints on particular portions of terraces within the crater, as in the 

 case of Theophilus, of which I present two drawings, one showing this 

 peculiarity in the morning light, the other not. The central mountains of 

 that great crater are high enough to throw long shadows ; and these, as 

 they catch upon other peaks or spread, softening with distance, over the 

 surrounding plains, present far greater variety of shadow-tones than might 

 be expected on a globe deficient, as the moon really appears to be, of 

 both air and water. 



The different parts of the moon's surface reflect light very unequally ; 

 the dark parts have several degrees of darkness, the light parts several 

 degrees of light. On the same level, as nearly as can be judged, under 

 the samic illumination, neighbouring parts are not only unequally reflective, 

 but their light seems to be of different tints. Within the large area of 

 Gassendi, under various angles of illumination, but more conspicuously 

 when the angle of incidence deviates least from verticality, patches of the 

 surface appear distinctly marked out by difference of tint, without shadow. 

 It is well known that in this particular photography has disclosed curious 

 and unexpected differences of the light, which were not aj)parent, or not so 

 obvious, to the eye. Reflecting telescopes seem to be indicated as most 

 suited for direct observation of differences of the kind of light on the moon. 



The surface of the moon is hardly anywhere really smooth, hardly any 

 where so smooth as may be supposed to be now the bed of a broad sea on 

 our globe. By watching carefully the curved penumbral boundary of hglit 

 and shade — as it passes over ridge and hollow, rift and plain, — broad swells, 



