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The whole is placed upon a stand, and shielded from reflected light, but fully exposed to that which is to be measured for actinic intensity. If now a constant current of electricity is passed through the apparatus, as indicated by a galvanometer, the variations of the latter will show the effect produced upon the selenium.
This power is also practically constant whatever the temperature of the air and is dangerous even on a cloudy day, when the heat waves are effectually screened off, but when the actinic rays are as active as ever. For this reason the protection of helmet and spine pad should never be omitted, no matter what the condition of the weather, between nine o'clock and four.
Those devils have got Evelyn!" He whirled upon Aten, seizing his shoulder, shaking the man as he roared questions. "No chance of catching them." Far away, in the jungle, the infinitely vivid actinic flame blazed for several seconds. "They've sprayed thermit on the road. It's melted and ruined. It'd take hours to haul the ground vehicles past the gap. They're got arms and lights.
Strangest of all, a phenomenon most marked in the children, Allan noticed that after a few weeks under the altered conditions of food and exposure to the actinic rays of the sun as reflected by the moonlight, pigmentation began to develop. A certain clouding of the iris began to show, premonitory of color-deposit.
The camps that were planned in iron and deal were built in stone and brass; the roads that were to have been mere iron tracks became spacious ways that insisted upon architecture; the cultivations of foodstuffs that were to have supplied emergency rations, were presently, with synthesisers, fertilisers, actinic light, and scientific direction, in excess of every human need.
These infinitesimally small ultra-violet or actinic waves, as they are called, are the principal agents in photography, and the great waves of wireless telegraphy are able to carry a force across the Atlantic which can sensibly affect the apparatus on the other side; therefore we see that the ether of space affords a medium through which energy can be transmitted by means of vibrations.
Hence a plate of tourmaline, with its axis parallel to the beam, stops the light, and with the axis perpendicular to the beam transmits the light. A plate of selenite placed between the Nicol and the actinic cloud shows the colours of polarized light; in fact, the cloud itself plays the part of a polarizing Nicol.
It is possible to make the particles of this actinic cloud grow from an infinitesimal and altogether ultra-microscopic size to particles of sensible magnitude; and by means of these in a certain stage of their growth, we produce a blue which rivals, if it does not transcend, that of the deepest and purest Italian sky.
A white actinic light shot from it caught us, bathed us. Snap had been awake; had heard the commotion of our encounter. His voice rang shrilly: "Stop! I'll shoot!" His warning siren rang out to alert the ship. His spotlight clung to us. Miko ran with me a few steps. Then he cursed and dropped me; fled away. I fell like a sack of carbide to the deck. My senses faded into blackness....
His most remarkable discovery, however, was that of the spiral character of the two Nubeculæ. With an effective exposure of four and a half hours, the Greater Cloud came out as "a complex spiral, with two centres"; while the similar conformation of its minor companion developed only after eight hours of persistent actinic action. The revelation is full of significance.
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