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Under the designation of "Mercurius Falsus" it received great attention from Schroter, who gives several representations of it in his Selenotopographische Fragmente, which, though drawn in his usual conventional style, convey a juster idea of its salient features than many subsequent drawings made under far better optical conditions.
Now, wherever optical continuity is ruptured we have reflection of the incident light. It is the multitude of reflections at the limiting surfaces of the particles that prevents light from passing through snow, powdered glass, or common salt. The light here is exhausted in echoes, not extinguished by true absorption.
Muybridge devised an instrument which he called a Zoopraxiscope for the optical projection of his zoetrope photographs. The succession of positives was arranged in proper order upon a glass disk about 18 inches in diameter near its circumference. This disk was mounted conveniently for rapid revolution so that each picture would pass in front of the condenser of an optical lantern.
It is simply an optical effect, a variety of the phenomenon called 'anthelia, like Ulloa's Circle and the famous 'Spectre of the Brocken." "Explain it how you will," rejoined Miss Carmichael, "to me it is an emblem of hope. It cheers my heart." "I am very glad to hear it, and I should be very sorry to crush your hopes," said Gazen pleasantly.
Only a few vapours, as thin as moonlight, fleeted rapidly across the stars. It was bitter cold; and, by a common optical effect, things seemed almost more definite than in the broadest daylight. The sleeping city was absolutely still; a company of white hoods, a field full of little alps, below the twinkling stars. Villon cursed his fortune. Would it were still snowing!
It was only my eyes that had been deceived; they had had a vision, one of those visions which lead simple folk to believe in miracles. It was a nervous accident to the optical apparatus, nothing more; the eyes were rather congested, perhaps.
Somewhat like Madeira, it has a longitudinal spine of mountains, generically called Las Canadas; but, whilst the volcanic ridge of the Isle of Wood runs in a latitudinal line, the Junonian Cordillera has a whorl, the ancient as well as the modern seat of eruption. Around the island appeared to be a rim, as if the sea-horizon formed a raised saucer a common optical delusion at these altitudes.
For the optical processes which occur on the surface of a mirror are such that, whilst taking place on a two-dimensional plane, they evoke in our consciousness pictures of exactly the same nature as if we were looking through the mirror into the space behind it.
But courage! though what we have seen was an optical illusion, there must be a real island in the distance beyond, of which this was the elevated and refracted image. It cannot, I think, be more than thirty or forty miles off, and the current is sweeping us steadily towards it."
When we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and graduated range of structure in the eyes of the lower animals; and when we bear in mind how small the number of all living forms must be in comparison with those which have become extinct, the difficulty ceases to be very great in believing that natural selection may have converted the simple apparatus of an optic nerve, coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the Articulata class.
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