92 Correspondence. 



otliers consist of elevated ranges and mountains, partly denuded, but 

 having tlieir summits strewn with snow and ice. Now, the reason 

 for determining this Glacial condition is this whiteness of all the 

 elevated portions, especially of the Polar regions, and of the peaks of 

 the highest mountains ; for if the Moon's surface were composed of 

 plutonic rocks, such as Granites, Basalts, Trajos, or covered with 

 volcanic products, such as cinders and lavas, this state would be 

 represented in the stereoscope by tints more or less neutral, and the 

 Moon's surface would not present that general white appearance in 

 the luminous portions which can only be due to reflection from an 

 uniform surface. The photograph, in fact, were the Moon's surface 

 composed of rocks uncovered by snow, or if it were clad by 

 vegetation, would not come out white at all. The stereograph of 

 Tycho is remarkably white, as much so at the base as at the summit. 

 Now, the peak of that mountain, which lies near the lunar pole, is 

 far above the snow line on the earth's surface ; and if there existed 

 any apparent atmosphere in the Moon, snow and ice would be 

 naturally looked for at that elevation, and the same would be 

 expected at the summits of our high mountans, as Copernicus and 

 Eratosthenes, and at the lunar poles. No one, in fact, whose eye has 

 been trained to the study of stereoscopic views of Snowy and 

 Glacial regions could, I think, fail to recognise the presence 

 of the same in the stereographs of Mr. De la Eue. The value of 

 photography appears to me to be this, that it determines, within 

 certain limits, the presence or absence of colour, and is to it 

 what chemical tests, or the spectrum analysis, are to matter. 

 The materials for comparison of the Moon's surface with the earth 

 ■even exist. Monte Viso, one of the Alpine range, has been seen by 

 the naked eye at a distance of 200 miles and the distinction between 

 snow and rock clearly made out. The Bernese Oberland has been 

 photogTaphed by Braun, of Domach, 60 miles off, and the elevated 

 chain of snowj'' peaks not only exactly resemble in colour those of 

 the elevated lunar mountains, but the distinction between rock and 

 snow is clear on the stereograph, and what is more, it is possible to 

 detect, at great distances, the difference between rock and shadow. 

 Tycho resembles a diminished Chimborazo, and although it is not at 

 present possible to bring Tycho nearer, it is available to remove Chim- 

 borazo optically as far off by taking stereoscopes of that and other 

 glacial moimtains with diminishiag lenses so as to place them at the 

 same relative distance, and they might then be usefully compared with 

 the lunar ranges. I have not gone into the question of the black 

 spots or patches about the lunar equator, which may be chasms, 

 frozen seas, or formations not having a reflecting surface, they 

 require to be the subject of future investigation ; but not only are 

 there glacial patches on them, but one of remarkable brilliance, 

 with light streaming in all directions, about the centre of the 

 Moon's equator, has a distiact crescent-shaped glacial ridge surround- 

 ing it. Nor do I here propose to enter into the reasons why the 

 Moon is glacial, whether owing to the cooling of its internal heat or 

 the unchecked radiation from its surface reflectiag the sun's rays 



