Lunar Delineation. 197 



satisfactorily obtained in the time wasted in some extensive 

 and disappointing attempt. 



Having chosen a region suitable for our purpose, we ought 

 obviously to sketch it under many varying angles of incident 

 light — soon after the lunar sunrise, more than once during the 

 advancing morning, at midday, and several times as the after- 

 noon declines, and towards sunset. This, however, partly 

 from weather, partly from the fact that the moon is not always 

 in an observable position at the required epoch, could not be 

 accomplished in a single lunation. Our whole series of studies 

 of any one spot must inevitably be made up of sketches taken 

 during different monthly periods, and perhaps after consider- 

 able intervals j and we must be prepared for discrepancies 

 arising from this cause. We may notice a slight difference in 

 the direction of the illumination, and, excepting near the 

 centre of the disc, the perspective foreshortening and relative 

 bearing, or what may be termed allineation, may be somewhat 

 altered ; and this may be accompanied with a change in the 

 visibility of minute details. Hence we shall feel inclined to 

 multiply our sketches, and to institute comparisons between 

 such as bear a general resemblance ; and thus we shall mate- 

 rially enlarge our knowledge of the true nature of the surface. 

 In order, however, to avoid mistake, the following considera- 

 tions must be borne in mind. 



Whatever similarity may exist at first sight, no strict com- 

 parison can take place between any two sketches in which the 

 angles of illumination and vision are materially unlike. If, 

 indeed, the representations taken under such different circum- 

 stances should be found to agree, one great point is gained : 

 we ascertain not only that the features are the same, but that 

 they are not of a character to vary much in appearance from 

 such causes. If, on the other hand, there should be much 

 discordance, we are still a long way from any safe inference as 

 to physical change. For while the angles of incident light 

 may be indefinitely varied, and frequently with little apparent 

 effect on the aspect of objects, there are cases in which a very 

 slight difference in the sun's altitude or azimuth would replace 

 brightness by a half tone, or even a black shadow ; and so 

 again a varied angle of vision, even if it does not, as may often 

 happen, affect the brightness of the surface more directly, may 

 influence our view of it materially by exposing w concealing 

 portions in light or shade. Either of these causes therefore, 

 or both conjointly, may account for much apparent incongruity 

 of representation. If the moon revolved around us in a cir- 

 cular orbit, in the plane of the ecliptic, and with an axis per- 

 pendicular to that plane, neither of these sources of discrepancy 

 would exist, but every portion of the surface would be illumi- 



