504 Changes in the Moon's Surface. [October, 



of the darkening of Plato which refers the phenomenon 

 to real processes of change, must be based on the conception 

 of physical processes unlike any with which we are familiar; 

 and it is a recognised rule in science that such conceptions 

 should be avoided. It is true that in regions to which we 

 are unable to extend experimental research such processes 

 may take place ; nay, we may say that certainly there occur 

 in nature many kinds of action which are unlike any we are 

 familiar with. But it is one thing to recognise such things 

 as possible or probable, another to accept them as legitimate 

 explanations of observed facts. Now, processes resembling 

 vegetation, recurring within a period of a few days, taking 

 place in an atmosphere more tenuous than the vacuum of a 

 receiver, repeated after a fortnight of intense cold, and 

 brought about by as remarkable an intensity of heat, must be 

 regarded as quite beyond our experience, and therefore 

 affording a very unsafe basis for reasoning, to say the least. 

 Nor is it a fact unworthy of being noticed that the apparent 

 maximum effect on the Floor of Plato occurs at the noon 

 hour of the place instead of at the hour corresponding 

 to two o'clock in the afternoon, when, according to our 

 experience, the accumulated effecl; of solar action is greatest. 

 Then, again, if we compare the darkening of Plato to some 

 of those chemical processes with which we are familiar as 

 effects of solar action, we find the change after lunar noon 

 at Plato unintelligible, since assuredly any chemical process 

 progressing as day advanced should not come to an end 

 at noon and then be replaced by the reverse process, but 

 would continue until the evening hours, and according to 

 our ordinary terrestrial experience would leave a permanent 

 effect. 



Setting aside other possible explanations — for it is a 

 mistake to suppose, as Mr. Birt appears to do, that surfaces 

 of different tints must maintain the same relative tints 

 under varying illumination — there is the effect of contrast 

 to be considered. This I believe, from my own observations, 

 to afford the true explanation of the observed phenomena. 

 Plato lies on lunar highlands, which shine very brilliantly 

 under high solar illumination. Towards the Mare Imbrium 

 a comparatively narrow ridge separates the floor from 

 that region. Now when the terminator has just passed 

 beyond Plato, the surrounding wall is not nearly so bright 

 as at the time of full moon ; the black shadow of its western 

 ridge occupies the western side of the floor ; and the eye, in 

 estimating the tint of Plato, is neither oppressed with the 

 glare of general light on the one hand, nor forced to 



