iS73-] Changes in the Moon's Surface. 507 



with varying position as libration operates to shift the 

 region. Now the aspect of the Floor of Plato, as seen with 

 a powerful telescope, suggests precisely that condition of 

 the surface which would render its appearance most likely 

 to be affected by such changes. The floor is in a general 

 sense level, though it does not suggest the idea of 

 smoothness. It looks as if it were granulated, and the 

 streaks and spots present the appearance of having a 

 surface not differing in tint alone, but in texture ; or rather 

 (such at least as been my own opinion when I have studied 

 the Floor of Plato with Lord Lindsay's 12^-inch telescope) 

 they suggest the impression that their difference of tone is 

 due much more to difference of surface-texture than to 

 difference of tint. Now we view the Floor of Plato very 

 obliquely, the line of sight having a mean inclination 

 of only 40 to the surface, and libration therefore affects 

 the direction of vision very importantly, according to 

 the principle indicated above. The range, in fact, roughly 

 is, from an angle of 33 to one of 47 , or the greatest and 

 least angles are nearly as 3 to 2. It would not be at all 

 wonderful, therefore, if a surface such as the Floor of Plato 

 varied greatly in appearance — now one, now another part 

 being the darker. 



The following experiment should be tried by those who 

 imagine that the unequal affections of such spots as are on 

 the Floor of Plato manifestly indicate real change. Let a 

 flat circular tin be filled with sandy earth, and over parts of 

 this earth let drops of different liquids be let fall, some 

 colourless, some slightly tinted, some drying with a glazed 

 surface, and so on. Let also certain spots be formed on the 

 surface, by removing portions of the sandy earth and 

 substituting finely crushed glass of various neutral tints. 

 Now let the general surface be viewed at an angle of about 40 

 (that is, the line of sight inclined 50 to the normal to the 

 surface) ; then (1) let the direction of illumination be varied, 

 while the direction of sight remains unchanged, and (2) let 

 the direction of the line of sight be changed through 6° or 

 7° on either side of the original angle of 40 . I venture to 

 affirm very confidently that the behaviour of some of the 

 spots will satisfactorily prove that apparent changes of 

 relative brightness are no sufficient evidence of real changes 

 in the nature or condition of a surface. 



The mistake seems to me also to have been made of 

 supposing that, because a lunar surface looks smooth, or 

 because the terminator when crossing the surface shows 

 no indentations, therefore the conditions of illumination 



