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There is only a little over half an octave that the eye can appreciate as light, and then all is darkness; but we can still go on further by the help of Science: beyond the violet we have the actinic or chemical rays, which are used in photography, and which enable us to trace the frequencies for a further two octaves.
In this ultra-violet region lie the X-Rays, and the other recently discovered high degree rays; also the actinic rays which, while invisible to the eye, register on the photographic plate, sunburn one's face, blister one's nose, and even cause violent explosions in chemical substances exposed to them, as well as act upon the green leaves of plants, causing the chemical transformation of carbonic acid and water into sugar and starches.
Mountain climbers cover their faces and hands with a mixture of grease and lamp-black in order to prevent sunburn. When in India we wore actinic underwear, dark glasses, and solar topees to protect us from the excessive light.
Each bump of his brain was the nucleus of a philosophical school. Hardly had he left the world, than the strong and simple light he shed was scattered in various hues by the prismatic minds that had surrounded him or that succeeded him; and in almost every case, as so often happens when the strands of the solar beam are brilliantly dishevelled, the actinic ray was lost."
"Humph!" thought Allan. "This may be a more serious factor than I've reckoned on. These people are albinos. White hair and pink eyes not a particle of protecting pigmentation. For thirty or so generations they've been subjected to nothing but torchlight. The actinic rays of the sun are infinitely more penetrating than anything they've ever known.
The instrument is used to measure the actinic power of sunlight when the sun is at various altitudes; but the same principle is applicable to other light sources. The sensitive part of the apparatus consists of a cylinder formed of 38 disks of copper, isolated from each other by as many disks of mica.
The naked eye is unaware of the existence of nebulæ in the Pleiades, or, at the best, merely suspects that there is something of the kind there; and even the most powerful telescopes are far from revealing the full wonder of the spectacle; but in photographs which have been exposed for many hours consecutively, in order to accumulate the impression of the actinic rays, the revelation is stunning.
She was there, standing near a huge isolated tree; Elza, pale, trembling, a hand pressed against her mouth in terror; disheveled, her garments dirty and torn with her wanderings through the forest. A swift glimpse as momentarily I paused; a second or two only, but the scene was impressed upon my brain as actinic light upon a photo-screen.
As a matter of truth, one may draw most of the values of the actinic rays of the sun through thin white clothing; and if one has not crushed his feet into a revolting mass in pursuit of the tradesmen, he may go barefooted a little while each day on his own grass-plot without shocking the natives or losing his credit at the bank.
In yellow stars, such as Capella and Arcturus, the same rhythmical series was partially represented, but associated with a great number of other lines; their state, as regards ultra-violet absorption, approximating to that of the sun; while the redder stars betrayed so marked a deficiency in actinic rays that from Betelgeux, with an exposure forty times that required for Sirius, only a faint spectral impression could be obtained, and from Aldebaran, in the strictly invisible region, almost none at all.
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