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



contrivance is employed to destroy the effect of contrast. 

 As it is conceded that none of the observed darkenings of 

 the Floor of Plato have been other than eye-estimates, no 

 further discussion seems needed, or can in my opinion be 

 legitimately given to the subject. I may mention, however, 

 that having, though as yet in an imperfect manner, studied 

 Plato with a much reduced field, so that I could eliminate 

 to some degree the effects of contrast,* I have not found 

 that the floor grows relatively darker towards the time of 

 full moon. 



The other circumstance which has been referred to 

 change, the variation of the visibility of the spots on the 

 Floor of Plato, cannot be explained as due to contrast, since, 

 in point of fact, the study of different spots introduces a 

 correction for any effects of contrast. Contrast might 

 make any given spot more conspicuous at one time than at 

 another ; but it could not cause one spot to be visible at 

 one time when another was invisible, and then after a time 

 the latter to be visible when the former was unseen. As 

 this is what has been detected in the spots on the Floor of 

 Plato, we must either believe that a real change takes place 

 in these spots, or else we must adopt some optical explana- 

 tion other than that depending on the laws of contrast. 



There is a difficulty in the assumption of real change in 

 these spots which does not present itself when we are 

 considering the change of a single crater, or of the floor 

 regarded as a whole. It is not easy to imagine any 

 processes by which different spots in the same limited 

 region of the moon, and under like circumstances, should 

 be very differently affected. If these spots were vegetation- 

 covered, we should expect them to show similar variations; 

 or if we admitted the possibility of different effects, resulting 

 from the same general process, we should still require 

 evidence showing that the changes of visibility had as a 

 period either the lunar day or the lunar year, neither of 

 which relations has in any single instance appeared ; and 

 similar results follow whatever real changes we imagine to 

 have affected these regions. 



But if we enquire into apparent changes taking place, we 

 find a case precisely corresponding to that of Linne. In 

 Linne the white spot assumes varying proportions and 

 dimensions with varying illumination, and doubtless also 



* In point of fact, the field was larger than Plato ; but I was able to get the 

 boundary of the field across the floor, and thus to produce effects corresponding 

 to those at sunrise and sunset, when there is shadow on the western or eastern 

 side of the crater. 



