ON MAPPING THE SURFACE OF THE MOON. 297 



the presence of colour is immistakeable, from a deep blackish grey to an 

 almost dazzling whiteness, but also with a gradation of brilliancy, some 

 spots sinning with a vividness far exceeding others, which, although they 

 may be regarded as " bright," are yet dull as compared with those that are 

 more vivid. Some of these spots are supposed to vary in intensity of 

 brightness, while others may be regarded as tolerably steady; and if some 

 amount of interest attaches to the smaller features of the moon's surface as 

 bearing upon the subject or question of change, no less amount of interest 

 will be found to characterize the range of brilliancy, especially when changes 

 which cannot altogether be referred to an alteration in the direction of the 

 incident rays, obtrude themselves on our notice. In the time of Schroter, if 

 not earlier, an attempt was made to chronicle these gradations of brilliancy 

 by adopting a decimal scale, in which the absolute absence of light was 

 regarded as zero, and the brightest spot on the moon's surface was denomi- 

 nated 10°. Schroter, Lohrmann, and Beer and Madler adopted this nota- 

 tion, and the first column, under the general heading " Brightness," is 

 intended to receive the results of a careful examination of all their estima- 

 tions, as well as the results of future eye-estimations of the brightness of 

 lunar objects. 



It is quite evident that estimations of any element can never command 

 that reception which instrumental determinations are able to do. With this 

 view a column has been inserted for receiving the measures of brightness or 

 reflective power, and it was thought that, by means of an instrument such 

 as that described by Mr. Dawes at the Meeting of the Eoyal Astronomical 

 Society, June 9, 1865 (see Monthly Notices, vol. xxv. p. 229), the gradations 

 of brightness might be measured with facility. The use of the instrument, as 

 recommended by Mr. Dawes, requires that the feature under examination 

 should be isolated by means of the solar eyepiece, thus shutting out all rays 

 from surrounding objects. Doubtless a graduated series of brightnesses may 

 be obtained in this way which would possess a great amount of value ; but 

 one feature has presented itself in some trials which were made to asee'rtain 

 the deepest tint that would quite obscure the brightest spot " Aristarchus." 

 It was obscured, and perfectly invisible, upon a ground of which the collective 

 power of reflexion was able to pierce the dark-tinted glass, and not only to 

 pierce it as a dull light, but also to show the outlines (somewhat indistinctly) 

 of the brighter and darker spaces on which the brighter spots then visible 

 were _ situated (not unlike the appearance of the night-side of the moon 

 illuminated by earth-shine a few days before and after change), the larger 

 darker spaces coming out with the same intensity as the larger brighter spaces, 

 so that upon settling the zero-point, the brighter spaces would be positive 

 and the darker negative ; thus while a zero might be obtained for a collective 

 reflective power, it would be diflicult to express by the same notation the 

 absolute brightness of the smaller brighter spots, or the absolute darkness of 

 the smaller darker spots. This being the case, it is possible a modification of 

 the homochromascope may be employed with advantage. (See Monthly 

 Notices of the Eoyal Astronomical Society, vol. xxii. p. 11.) 



One very important circumstance that may present itseif to the observer 

 and one that will assume a much greater degree of importance in the estima- 

 tion of the selenographer, engaged in symbolizing and cataloguing objects is 

 the unmistakeahle identification of the smaller features, such as minute craters 

 mountains, rills, soft ridges, &c. A simple mode of recognition and identifi- 

 cation presents itself in the alignment of objects, and for this two objects one 

 on each side of the object to be identified, will suffice. If, however, it should 



