1873O Condition of the Moon's Surface. 53 



floor of Plato towards the terrestrial observer is affected by 

 libration, now tilting the floor more fully towards the ob- 

 server and presently tilting it away from him ; at one time 

 tilting the floor eastwards, at another westwards, and at 

 intermediate periods giving every intermediate variety of tilt, 

 — these changes, moreover, having their maximum in turn 

 at all epochs of the lunation. Combining this consideration 

 with the circumstance that very slight variations in the pre- 

 sentation of a flattish surface will cause certain portions to 

 appear relatively dark or relatively light, it appears to me 

 that a case has not yet been made out for those seleno- 

 graphical changes by which Mr. Birt has proposed to in- 

 terpret these phenomena. 



Nevertheless it cannot be insisted on too strongly that it 

 is from the detailed examination of the moon's surface that 

 we can now alone hope for exact information as to its 

 present condition and past history. I would even urge, in- 

 deed, that the detailed examination at present being carried 

 out is not sufficiently exact in method. I should be glad 

 to hear of such processes of examination as were applied by 

 Mr. Dawes to the solar spots. In particular it seems to me 

 most important that the physiological effects which render 

 ordinary telescopic observation and ordinary eye-estimates 

 of size, brightness, and colour deceptive, should be as far 

 as possible eliminated. This might be done by so arranging 

 the observations that the conditions under which each part 

 of the moon should be studied might be as far as possible 

 equalised during the whole progress of the lunation. Thus, 

 returning to the case of the floor of Plato : this region 

 should not be examined when Plato is near the terminator 

 as well as at the time of full moon, with the rest of the 

 moon's disc or large portions thereof in the field of view ; 

 the eye of the observer should be protected from all light 

 save that which comes from the floor itself; and, moreover, 

 the artificial darkness produced for this purpose should be 

 so obtained that the general light of the full moonlight 

 should be excluded as well as the direct light from the disc. 

 Then differences of tint should be carefully estimated either 

 by means of graduated darkening-glasses, or by the intro- 

 duction of artificially illuminated surfaces into the field of 

 view for direct comparison with the lunar region whose 

 brightness is to be determined. 



When observations thus carefully conducted are made, and 

 when the effects of libration as well as of the sun's altitude 

 above the lunar regions studied are carefully taken into 

 account, we should be better able tfyan we are at present, as 



