Now if this ingenious mode of procedure is to be rendered of any practical avail, advantage must, above all, be taken of the finer air of the mountains. This for two reasons. First, because the glare which, as it were, smothers the delicate structure we want to obtain records of, is there at a minimum; secondly, because the violet rays by which it impresses itself upon the “photographic retina”[30] are there at a maximum. These, as Professor Langley’s experiments show, suffer far more from atmospheric ravages than their less refrangible companions in the spectrum; the gain thus to them, relatively to the general gain, grows with every yard of ascent; the proportion, in other words, of short and quick vibrations in the light received becomes exalted as we press upwards—a fact brought into especial prominence by Dr. Copeland’s solar observations at Vincocaya, 14,360 feet above the sea-level. Indeed, for all the operations of celestial photography, the advantages of great altitudes can hardly be exaggerated; and celestial photography is gradually assuming an importance which its first tentative efforts, thirty-four years ago, gave little reason to expect. Thus, in three leading departments of modern astronomy—solar physics, stellar spectroscopy, and the wide field of photography—the aid of mountain observatories may be pronounced indispensable; while in all there is scarcely a doubt that it will prove eminently useful. There are, indeed, difficulties and drawbacks to their maintenance. The choice of a site, in the first place, is a matter requiring the most careful deliberation. Not all elevated points are available for the purpose. Some act persistently as vapor-condensers, and seldom doff their sullen cap of clouds. From any mountain in the United Kingdom, for instance, it would be folly to expect an astronomical benefit. On Ben Nevis, the chief amongst them, a meteorological observatory has recently been established with the best auguries of success; but it would indeed be a sanguine star-gazer who should expect improved telescopic opportunities from its misty summit.