258 REPORT— 1872. 



reflect more liglit, they -would stand out as luminous streaks and markings, 

 to be partially or' wholly obscui'ed by any condensed vapour which may 

 happen to be floating above them. An absence of condensed vapour would 

 impart to the floor a sharpness and definiteness so often observed, by the aid 

 of which very minute objects are easily seen, while even a slight film, ana- 

 logous to our cirrostratus cloud, would impart a mistiness by which the more 

 delicate spots and streaks would be obscured, the broader features still re- 

 maining visible — a state of things of which evidence exists in the observations, 

 and which has been observed more or less since the time of Ilevelius, who 

 relates that " several times he found, in skies perfectly clear, when even stars 

 of the sixth and seventh magnitudes were visible, that at the same altitude 

 of the moon, the same elongation from the earth, and with one and the same 

 telescope the moon and its maculas did not appear equally lucid, clear, and 

 conspicuous at all times, but were much brighter and more distinct at some 

 times than at others." 



In a letter written under date of September 27, 1871, the Eev. T. "W. 

 "Webb suggests that there is more inequality than we have yet studied in 

 the reflective power of diff"erent substances at different augles of illumination 

 or incident light. " It may be," he says, " that different colours behave 

 differently when treated in this way ; and I suspect they do : ccj., if from the 

 surfaces of certain materials the capacity for reflection of blue rays should 

 increase more rapidly than that for red, then with increasing angle of illumi- 

 nation the colour of the object would slightly change, and with it its propor- 

 tional visibility at a distance where colour becomes impercejitible. * * * Is 

 it not possible that other circumscribed regions in the moon, e. cj. that 

 glorious Archimedes, might show variations in the markings even more defi- 

 nite and considerably more easily dealt with than those in Plato? The 

 curiously but occasionaUij speckled and streaky aspect of the Mare Crisium, 

 as described by Schroter, E. & M., and others, would be a grand case were 

 it not so rare. If we could only find some smaller and more easily studied 

 surface, equally or more frequently varied, it would be a great matter. But 

 is there not something of a more general character underlying, as it were, 

 these special instances that has never yet been properly investigated ? "We 

 lay it down, as if it were unquestionable, that local colour in the moon is 

 masked in the rising and setting illumination, and comes out under high 

 angles when the shadows disappear. It may be so ; but why ? If I took a 

 piece of plaster of Paris, moulded it into all sorts of hollows and knobs, and 

 painted it harlequin fashion, then the colouring would be all equally visible, 

 whether under oblique or vertical illumination; or, to make it more like the 

 moon, if the artificial surface were only shaded with brownish or bluish greys 

 we should have the same effect ; as long as there was light enough to show 

 it, the distinction of colour would remain. On the moon it is fiir less evi- 

 dent, and frequently quite imperceptible. Now what imderlies this ? "^'hy 

 do certain very dark spots on the moon come out under high illumination, 

 or certain brilliant specks, being much less if at all contrasted with the 

 neighbourhood when near the terminator? Could we produce an artificial 

 surface which would behave in the same way? 11 /'?/ should this difference 

 (whatever may be its cause) depend, not on the angle of incidence, but on 

 that of reflection ; for the full-moon aspect extends over the whole disk, not- 

 withstanding the low illumination of the regions all round the limb, many of 

 which show spots as vivid or as deep as more central regions? No one, so 

 far as I know, has touched at all on this very interesting point." 



Mr. Webb's suggestions and queries are very important. On that of an 



