^7 3-1 Changes in the Moon's Surface. 503 



used in the observations of Plato, so far as I know. What 

 I wish to indicate is that, at the beginning of this inquiry, 

 Mr. Birt recognised the necessity of some instrumental con- 

 trivance for determining colour tints, as distinguished from 

 mere processes of eye-estimation. 



The observations of the Floor of Plato divide themselves 

 into two classes. First, there is the observation of the floor 

 itself, regarded as a whole, and compared with neighbouring 

 regions, and especially with the neighbouring parts of the 

 Mare Imbrium; then, secondly, there is the study of the 

 spots, thirty-seven in number, which have been detected in 

 the floor, and which vary in visibility, not only absolutely 

 but relatively. 



The main result which Mr. Birt deduces from the general 

 study of the Floor of Plato is that it grows darker as the 

 lunar day advances, being darker near noon at Plato. I do 

 not know that any good would be gained by entering into 

 minute details, which would occupy much space, and would 

 not elucidate the subject, since those who believe that a real 

 change takes place attach no importance to these minutiae, 

 but ask for acceptance only of the general fact that as the 

 sun rises higher above the level of Plato the floor of 

 the crater grows darker as compared with the neighbouring 

 region, and that the morning tints are resumed as the sun 

 gradually passes from its culmination descendingly towards 

 the western horizon of Plato. 



I have before me as I write a series of observations made 

 by Mr. Neison, F.R.A.S., which would occupy, were I to 

 quote them in full, far more space than remains available to 

 me. They indicate, as satisfactorily as need be, the process 

 of darkening to which I have just referred. 



Nothing could seem more clearly demonstrated than the 

 fact that Plato darkens towards the noon-tide hours of that 

 lunar region. We might enter on the inviting speculations 

 suggested by such a state of things, inquiring whether that 

 darkening was due to some process of vegetation, or to 

 chemical changes in the surface of the floor. And a variety 

 of speculations more or less ingenious might suggest them- 

 selves as to the general condition of the regions within the 

 circular crater-walls. 



But we must not overlook the possibility that the 

 darkening of Plato may be apparent only, not real. It 

 is most important that before we begin to reason on 

 processes of change we should assure ourselves that change 

 really does take place. There is another reason for caution in 

 the circumstance that, so far as can be seen, any explanation 



