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minator as well as at the time of full moon, with therest of the moon’s disc or large portions thereof in thefield of view; the eye of the observer should be pro-tected from all light save that which comes from thefloor itself; and, moreover, the artificial darknessproduced for this purpose should be so obtained thatthe general light of the full moonlight should beexcluded as well as the direct light from the disc.Then differences of tint should be carefully estimatedeither by means of graduated darkening-glasses, orby the introduction of artificially illuminated surfacesinto the field of view for direct comparison with thelunar region whose brightness is to be determined.
When observations thus carefully conducted aremade, and when the effects of libration as well as ofthe sun’s altitude above the lunar regions studied arecarefully taken into account, we should be better ablethan we are at present, as it appears to me, to deter-mine whether the moon’s surface is still undergoingchanges of configuration. I cannot but think thatsuch an inquiry would be made under more promisingcircumstances than those imagine who consider thatthe moon’s surface has reached its ultimate condition,and that therefore the search for signs of change is ahopeless one. So far am I from considering it unlikelythat the moon’s surface is still undergoing change,that, on the contrary, it appears to me certain thatthe face of the moon must be undergoing changes ofa somewhat remarkable nature, though not pi'oducingany results which are readily discerned by our imperfect