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Along with this I purchased every possible accessory, drawtubes, micrometers, a camera-lucida, lever-stage, achromatic condensers, white cloud illuminators, prisms, parabolic condensers, polarizing apparatus, forceps, aquatic boxes, fishing-tubes, with a host of other articles, all of which would have been useful in the hands of an experienced microscopist, but, as I afterwards discovered, were not of the slightest present value to me.
The difference of a minute fraction of an inch in the thickness of the "rim" would break extractors as fast as they could be replaced, while various other irregularities, so small as to be undiscoverable without the most accurate measurements by delicate micrometers, would cause stoppages and the breaking of different small parts.
As telescopes increased in power, however, it was found that even the finest wire, or silk filaments, were much too thick for astronomical observations, as they obliterated the image, and so, finally, the spider-web came into use and is still used in micrometers and other similar instruments. Before that time, however, the fine crossed wires had revolutionized astronomical observations.
The micrometers of Malvasia, and later of Auzout and Picard, are the development of this discovery. Malvasia's micrometer, which he described in 1662, consisted of fine silver wires placed at right-angles at the focus of his telescope.
Behind them there is a beautiful park, which extends over the undulating and rising ground in the rear; and on the summit of one of the eminences there is the famous Greenwich Observatory, on the precision of whose quadrants and micrometers depend those calculations by which the navigation of the world is guided.
Tycho's instruments were necessarily most rudely divided, and he had, of course, no telescopes to aid him. Consequently it was merely by a process of sighting that he could obtain the places of the stars. It must further be remembered that Tycho had no clocks, and no micrometers. He had, indeed, but little correct knowledge of the motions of the heavenly bodies to guide him.
The Colors of Striated Surfaces Young similarly examined the colors that are produced by scratches on a smooth surface, in particular testing the light from "Mr. Coventry's exquisite micrometers," which consist of lines scratched on glass at measured intervals. These microscopic tests brought the same results as the other experiments.
When, in the Bank of England, I see a wondrously delicate machine for testing sovereigns, a shrewd implacable little steel Rhadamanthus that, once the coins are delivered up to it, lifts and balances each in turn for the fraction of an instant, finds it wanting or sufficient, and dismisses it to right or left with rigorous justice; when I am told of micrometers and thermopiles and tasimeters which deal physically with the invisible, the impalpable, and the unimaginable; of cunning wires and wheels and pointing needles which will register your and my quickness so as to exclude flattering opinion; of a machine for drawing the right conclusion, which will doubtless by-and-by be improved into an automaton for finding true premises; of a microphone which detects the cadence of the fly's foot on the ceiling, and may be expected presently to discriminate the noises of our various follies as they soliloquise or converse in our brains my mind seeming too small for these things, I get a little out of it, like an unfortunate savage too suddenly brought face to face with civilisation, and I exclaim
"We had better also take a microscope and the micrometers, though we are not likely to want them; that is all, I think." "A box and a portfolio," repeated Polton in a speculative tone. "Yes, sir, I will take them with me." He opened the door and was about to pass out, when, perceiving a visitor ascending the stairs, he turned back. "Here's Mr. Miller, from Scotland Yard, sir; shall I show him in?"
The magnification is eight diameters, so that the squares are here each two-thirds of an inch in diameter. I have a number of these micrometers of different scales, and I find them invaluable in examining cheques, doubtful signatures and such like. I see you have packed up the camera and the microscope, Polton; have you put in the micrometer?"
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