336 
PROFESSOR PHILLIPS OJSf SOME PARTS 
level, as nearly as can be judged, under the same 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 apparent, 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 anywhere so smooth 
as may be supposed to be now the bed of a broad sea on our globe. By watching care- 
fully the curved penumbral boundary of light and shade, — as it passes over ridge and 
hollow, rift and plain, — broad swells, minute puckerings, and small monticules appear 
and disappear in almost every part. In several of the maria, minute annular cup-craters 
about half a mile across are frequent ; and on several of the exterior slopes of the 
crater-rings occur pits, ridges, fissures, and rude craters, something like the sloping sur- 
faces of Etna. Copernicus is a good example of this common occurrence. It appears 
extremely desirable that the details of this magnificent mountain should be carefully 
reexamined on the basis of Secchi’s fine drawing, for the purpose, amongst others, of 
determining exactly how many of the bosses and ridges bear cup-hillocks ; for many 
inequalities, which in feeble telescopes have but the indistinct character of being heaped 
up, appear distinctly crateriform with superior optical power*. 
For the purpose of determining the true form of the summits, and the outline of 
the sides of many mountains, the method of observation of the shadows at different angles 
of the incident sunlight will be found very useful. Thus, exactly as, in the clear evening 
hours, one standing on the summit of the Malverns sees the long shadow sweep over the 
vale of the Severn, and distinguishes the forms of the several beacons and intervening 
passes, so in the clearer moonlight, the shadows which fall within the craters, and stretch 
along the plains, often reveal the presence of angular escarpments and deep fissures as 
well as of peaky summits crowning steep walls of rock. 
One of the circumstances which is thus often placed in evidence is the exceeding 
abruptness of many edges, and the uncommon steepness of many of the slopes. In par- 
ticular, the interior edges of many ring-mountains appear with violently precipitous 
cliffs and chasms, more or less parallel to the general outline ; while, more rarely, deep 
cross cuts in such situations appear to radiate within the ring, or to traverse its crest. 
An instance of the latter kind is found in Gassendi. 
The steepness of the moon-crater walls and slopes is much greater in general than 
in any, except very rare examples, known among the volcanic regions of the earth. 
Popular descriptions are, in this respect, very misleading ; and the word-paintings which 
please so much the gratified ear, lose their reputation for exactness when confronted by 
* See “ Comparative Remarks on Gassendi and Copernicus,” Roy. Soc. Proc. for 1856, p. 74. 
