Lunar Mountains Tyclw, Pictet^ and Saussure, 287 



visible as luminous points in the interior of the bands ; the steepness of the 

 slope, more or less, has no influence on this matter ; and there is in fact no 

 criterion of the visibility of these objects to bo discovered when the moon is 

 atthefuU.* 



About this period, the central mountain of Tycho is a clearly determined 

 luminous point, of 8° of brightness. The whole circumference exhibits a 

 low flat ; a part of the lower terraces which surround it in two places near 

 the eastern zone are at 5" of brightness, and the higher terraces are at 3* 

 only.t The grand 'principal zone, about half a Gennan mile broad, is at 

 8% and this is the only one of this extent, to a distance of at least fifty miles, 

 which shews itself by its brightness ; and it is this circumstance which con- 

 fers on Tycho its very striking appearance during the full moon. 



A grey zone of 3* of brightness towards the outer base of the inclosure, 

 and somewhat more luminous beyond, predominates all round this zone, to 

 a distance of about five German miles, and it is this space like a dark 

 crown, which Hevelius on his map denominated Descrtum Zin. To its 

 W.SW., at the distance of two miles from its margin, a small luminous 

 point is perceptible, which is probably the point B ; all the rest is a smooth 

 surface. 



It is from this grey zone, as it becomes more luminous, that there suc- 

 cessively rise the bands which have been mentioned above, some proceed- 

 ing from it directly, as that which runs towards BuUiald, whilst the greater 

 number at first are confounded in a luminous areola, which, in some places* 

 extends to a breadth of twenty German miles. As soon as the bands ap- 

 pear distinctly separated, we perceive between them, and even upon them, 

 the luminous points in small numbers, of which we have already spoken. 

 The majority of the craters have a brightness of from 6° to 7°, and the 

 bands themselves from 5" to 7*. 



In the north, to an extent of sixty German miles, only four distinct spots 

 are to be found, and this not without much difficulty. Near one of them 

 a whitish luminous spot may be found, at 9° brightness, which appears to 

 proceed from the concourse of numerous bands. It is situated, so far as we 

 have been able to judge, in quite a flat region, in 33° lat. and 3|° long. 

 This white spot is nothing less than the whitish cloud which was perceived by 

 Cassini in the year 1671, which he proclaimed as a great discovery, and in 

 place of which he saw in 1673 a new large spot. At the present day, 

 an attentive observer may see a whitish cloud appear four days before 



* Dr Madler has given, in Astr. Nachr. No 283, by means of a copperplate 

 impression, two representations of the region Tycho, on a single sheet, and on 

 a scale double the size of the general map. In the one of these, the district is 

 viewed in an oblique light ; in the other as seen at full moon. 



•f- This singular circumstance, of the terraces being more obscure than the 

 cavity, appears to have led some observers to the conclusion, that the interior 

 of Tycho is convex. But internal surfaces, which are truly convex in annular 

 mountains, such as Mersenius, Petavius, and Hevelius, manifest their shape in 

 quite a different way than by a greater degree of brightness ; and it is always im 

 possible to conclude directly the height of any point of the moon from the inten- 

 sity of its light. 



u 2 



