﻿relative to Lunar Activity, 185 



ought to have, after a sufficient interval for determining normal 

 visibilities, a regularity of the phenomena observed ; for example, 

 the curves ought to present regular maxima and minima corre- 

 sponding to changes of illumination, reflection, and visual ray, 

 if these agencies are capable of affecting visibility. And as re- 

 gards the earth's atmosphere, which does affect in no small 

 degree " visibility," the whole of the spots, we might suppose, 

 would present similar inflexions of their curves ; indeed the area 

 of Plato is so small in comparison with that of the disk, that it 

 is difficult to conceive how any of the spots can be differently 

 affected by changes in our own atmosphere, further than that those 

 within reach of vision under all circumstances of change in our 

 atmosphere would exhibit at all times nearly the same degrees 

 of visibility, while small and faint spots (seen only in the finest 

 weather and with the largest apertures) would, as the observa- 

 tions proceed, continually decline in visibility. 



In the first trial, purely tentative as it was, the curves of spots 

 Nos. 19, 5, 13, 14, 22, 7, and 16 were compared, and it was 

 found that they presented well-marked acuminated maxima in 

 August and September 1869 (see Plate III.). These, with the 

 exception of No. 7, are situated on the western part of the floor. 



In addition to the spots on the floor of Plato, there are certain 

 markings, as represented in the accompanying sketch (fig. 1). 



Fig. 1. 



These markings, which are very variable in visibility and intensity, 

 are given of a uniform whiteness. There is reason to believe that 

 some have disappeared ; indeed it is not expected that on any 

 occasion the floor can be seen as represented in the sketch ; it is 

 only intended to indicate those portions which during the last 

 fifteen years have been free from markings; those on the sketch 

 have all been seen since the beginning of 1860, and by far the 

 greatest number within the last two years. Four light streaks 

 crossing the floor from north to south are given in the first edi- 

 tion of Beer and Madler's large map, and mentioned in Der 



