1873O Condition of the Moon's Surface. 51 



green, while the greater photographic power of the parts 

 under full solar illumination may indicate an increase of 

 green light due to some process of vegetation. It is, how- 

 ever, important to inquire whether the greater part of the 

 difference may not be due to a physiological cause ; whether, 

 in fact, the neighbourhood of the dark portion of the disc 

 may not cause the illuminated parts near the terminator to 

 appear, through contrast, brighter than they really are. 



On the answer which may be given to this question de- 

 pends, in a great degree (as it seems to me), the opinion we 

 are to form of those recent researches by Mr. Birt which 

 have appeared to indicate that the floor of Plato grows 

 darker as the sun rises higher above it. Taking these re- 

 searches in their general aspect, it cannot but be recognised 

 that it is a matter of the utmost importance to determine 

 whether they indicate a real change or one which is only 

 apparent. If it is really the case that Plato grows darker 

 under a rising sun, we should have to infer that in the case 

 of Plato certainly, and probably in the case of other regions 

 similarly placed, processes of change take place in each 

 lunation which correspond (fairly) with what might be ex- 

 pected if these regions became covered with some sort of 

 vegetation as the lunar month (or, which is the same thing, 

 the lunar day) proceeds. Other explanations — meteorolo- 

 gical, chemical, or mechanical — might indeed be available, 

 yet in any case conclusions of the utmost interest would 

 present themselves for consideration. 



It must be remembered, however, that thus far Mr. Birt's 

 observations (as well those made by himself as those which 

 he has collected together) are based on eye-estimations. 

 Nothing has yet been done to apply any photometric test to 

 the matter ; nor has the floor of Plato been brought alone 

 under observation, but other light, of varying degrees of 

 intensity, has always been in the field of view. Plato is 

 seen bright when near the " terminator," and growing 

 gradually darker as the sun rises higher and higher above 

 the level of the floor of the crater. The point to be decided 

 is, how far the brightness of Plato near the terminator is 

 an effect of contrast. De la Rue's photographic observa- 

 tions go far to prove (they at least strongly suggest) that 

 contrast has much to do with the matter. He has shown 

 that, photographically, the parts near the terminator are not 

 so bright as they look. May it not be that they look brighter 

 than they are in reality ? We have only to suppose that 

 De la Rue's photographic results represent pretty accurately 

 the true relative luminosity of different parts of the moon 

 to answer this question at once in the affirmative. 



