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But there is an instrument of very simple construction, called a "quarter-undulation plate," a plate usually of mica, whose thickness is an odd multiple of a quarter of a wave-length, which enables us to discriminate between light unpolarized and circularly polarized.
Two and a half octaves above these numerically, i.e. at about the twentieth octave, we reach the frequency of Electro-Magnetic Rills, used by the Marconi System of wireless telegraphy, which pulsate at about 950,000 per second, and have a wave-length of something like 1000 feet.
Well, he found out something funny about the way everything acted while the Mardonalians were bombarding them with a certain kind of a wave-length.
Supposing a certain thickness of the gypsum produces a retardation of half a wave-length, twice this thickness will produce a retardation of two half wave-lengths, three times this thickness a retardation of three half wave-lengths, and so on.
At the edge, however, he stopped the brink of a precipice, to him! and Spinrobin then became aware that from his moving lips, doll-like though bearded, his voice was issuing with an ever-growing volume of sound and power. Vibrations of swiftly-increasing depth and wave-length were spreading through the air about him, filling the room from floor to ceiling.
Not only is this the case, but the actual dynamic impulse of the new life which rouses at puberty is alien to the original dynamic flow. The new wave-length by no means corresponds. The new vibration by no means harmonizes. Force the two together, and you cause a terrible frictional excitement and jarring.
It is produced by an obliquity of direction which causes the paths of the marginal waves to be a whole wave-length different from each other. When the difference between the paths of the marginal waves is half a wave-length, a partial destruction of the light is effected.
This ensures that they can be observed clearly by the human eye. Much smaller objects, however, will require a correspondingly shorter wave-length in the medium of observation. Now shorter wave-lengths than those of visible light have been found in ultra-violet light and in X-rays; and these, accordingly, are now often used for minute physical research.
Now, when the Nicols are parallel, the retardation of half a wave-length, or of any odd number of half wave-lengths, produces extinction; at all thicknesses, on the other hand, which correspond to a retardation of an even number of half wave-lengths, the two beams support each other, when they are brought to a common plane by the analyzer.
I hoped you'd call me. Didn't want to chance it, raising you when you might not be alone." I told him swiftly what I had done; where I was now. And Hanley said, with equal briskness: "I've an important fact. Just had Markes on secret wave-length. He tells me that Spawn has been saving up his quicksilver for six months past.
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