in certain regions, and in consequence of increasing vapours, it altogether changes to a very pale blue.
156.
The mountains, in like manner, appear to us blue; for, as we see them at so great a distance that we no longer distinguish the local tints, and as no light reflected from their surface acts on our vision, they are equivalent to mere dark objects, which, owing to the interposed vapours, appear blue.
157.
So we find the shadowed parts of nearer objects are blue when the air is charged with thin vapours.
158.
The snow-mountains, on the other hand, at a great distance, still appear white, or approaching to a yellowish hue, because they act on our eyes as brightness seen through atmospheric vapour.
159.
The blue appearance