OBSEllVATIONS OF LUNAR OBJECTS. 261 



reflection of ]ig]it, in connexion with the above remark I may notice that an 

 increase of light from dark sLatcd roofs is observed as the sun approaches 

 such a position in the heavens, with regard to the eye, that if the roofs con- 

 sisted of polished surfaces an image of the snn would be seen at the moment 

 when the angle of reflection equalled the angle of incidence ; and this tendency 

 to the formation of an image is greatlj' augmented by a falling shower, the 

 rain bringing the slates more into tlie condition of a watery surface, rendering 

 them comparatively darker, except at those points where the two angles meet. 

 Irregular and roughened surfaces on the moon will consequently appear 

 brighter at those periods in the lunation when the light from the sun to the 

 moon, and thence to the eye, falls in the lines of incidence and reflection. 

 At aU times irregular reflection from the moon is independent of the incident 

 rays ; but an approach to regular reflection attended with increase of light, 

 the epoch of which for each point of the moon's surface is clearly calculable, 

 must occur during every lunation, so that all normal brightening may be 

 easily detected.] 



" Starting from the hypothesis, it seems to me that your explanation of the 

 variation of position of the Jiglit-streahs must be held a very reasonable one ; 

 and I beg to add a small supplement. I suppose that the maxima of midday 

 heat at the latitude of Plato are not always the same, that they run through 

 a slight seasonal variation, then a periodical diflerence in the quantity of 

 vapour raised would result ; and if the general outline of the light-streaks is 

 the result of the local conformation of the ground as related to the craterlets, 

 then an unusual amount of vapour raised; might cause two or more light- 

 streaks apparently to flow together for a time, producing an entire change in 

 their outline, afterwards, as the heat lessened, to resume their most usual 

 appearance. I have often thought the light-streaks suggested the remem- 

 brance of the mists and fogs which may be seen on an autumnal evening 

 from the elevated parts of our downs ; as evening advances the mists gradu- 

 ally appear winding about in streaky shapes or isolated in irregular patches, 

 according to the formation of the ground, while their margins, sometimes 

 sharp, are generally soft and ill-defined, very much like lunar light-streaks 

 to my mind, if they could be seen from a nearly perpendicular direction. 



"There is a difiiculty as to the nature of the dark tints, supposing them to 

 be cloudy coverings; but is it quite certain that the middle tint is reaUy the 

 true colour of the soil? What if the darkest tint was the colour of the 

 naked surface, and if the middle tint, which appears at sunrise and sunset, 

 is a covering of the nature of hoar frost, the vapour which is supposed to be 

 raised during the luni-solar day freezing again on the sudden approach of 

 night ? But we are supposing the presence of moisture on the moon. Wc 

 must not forget the Baconian maxim." 



[Mr. Pratt's idea of hoar frost is very suggestive ; not that the appearance 

 of the surface near the terminator is of that nature in the sense in which we 

 employ the term, for one would think that then the reflected light would be 

 most certainly white ; yet it cannot be denied that, generally speaking, within 

 about 10° of the terminator, the surface of the moon is such as wo may 

 suppose that of the earth to present after a cold clear frosty night, the atmo- 

 sphere being exceedingly translucent, so that objects are readily and clearly 

 visible. The lunar night must be intenselj^ cold, with enormous radiation, 

 by which, whatever gas or vapour may exist, both its bulk and elasticity 

 would be greatly reduced, especially towards sunrise ; so that it is quite 

 possible that an atmosphere, if such there be, would be of the rarest charac- 

 ter, and this would fully account for the clearness and sharpness of objects at 



