OBSERVATIONS OF LUNAR OBJECTS. 251 



branch, and also as the floor becomes darker as the sun ascends higher, 

 an ordinary light tint has been fixed at 0-33, and a dark tint at 0-66 ; very 

 light and very light indeed have been registered i)rovisionally lower than 

 0-33, and very dark and very dark indeed higher than 0-66, so as to give a 

 range, as regards Plato, of 1-00. The actnal range resultiug from 133 obser- 

 vations in two years is 0-41, and the range of solar altitude at the equinoxes 

 on the parallel of 50° is 40°. The chromatic range very nearly coincides 

 with that of altitude, and the connexion between the tint of the floor and 

 the effect either of light or heat is plain and unmistakable. The floor 

 must therefore consist of material capable of becoming darker by exposure 

 to light and heat, or it must possess a covering that may possibly be affected 

 in the same way. The inflexions of the chromatic curve indicate rather 

 considerable variability, especially in the deepening of the tint, which 

 hardly accords with a permanent surface being heated by definite and 

 regular increments of heat : and it would also appear that the solar effect is 

 not fully attained ; for although the ranges of both curves are very nearly 

 equal, a mean chromatic curve drawn with a free hand would indicate an 

 average liffhter floor than that which a regular heating might be expected to 

 produce. So far as the writer is aware, this is the first attempt to indicate 

 numerically the chromatic effect of light or heat, or both, upon the moon's 

 surface. It has long been known that the grey plains appear darkest under a 

 high sun, but the knowledge of the nature of the progression has been vague 

 and undefined. It is greatly to be desired that other spots, especially in 

 lower latitudes, should be observed in the same way ; but some time must 

 necessarily elapse before observations of them could be compared with those 

 of Plato. 



II. 

 An Examination of Changes recorded in August 1869. 



These changes were recorded in four carefully executed drawings of the 

 floor of Plato by Mr. Pratt. They exhibit, first, a rapid alteration of the 

 distribution of the light and dark portions of the floor between the 16th and 

 17th of August, and a more gradual but slight, yet still perceptible, change 

 from the 17th to the 28th. Calling the figures in their order 4, 5, 6, and 7, 

 and starting with the assumption that the permanent colour of the floor is 

 light (see Section on the influence of the sun on the floor of Plato), we may 

 trace the changes between each of the observations, remarking at the outset 

 that the spots are presumed to be permanent as to their positions on the 

 floor. 



. Fig. 4, August 16, 1869, exhibits a disposition of the darker shading 

 entirely detached from the border on every side. The shape is roughly that of 

 aW, — the western leg being the widest, with spots Nos. 14, 1, and 19 just on 

 its border ; the dark space forming the middle leg extending from beyond 

 No. 17 to beyond No. 3, both spots being involved in it; and the eastern 

 leg very near the east border, having spot No. 6 on its western edge. Seven 

 spots are given on the drawing, viz. Nos. 14, 1, 6, and 19, just on the border 

 of the darker portion, Nos. 3 and 17 in its midst, and No. 4 on the Light 

 portion. 



Theorizing merely as a help to connect and interpret the phenomena 

 observed, and assuming that the lighter tint is that of the floor and the dark 

 tint that of a something which varies in position, the nature of which we 

 have yet to learn, we have in fig. 4 its disposition on August 16, 1869. Of 

 the shading of the floor on this day Mr. Pratt thus writes : — " This was more 



