Visual practices across the university, part two, chapters 1 16
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Visual practices across the university, part two, chapters 1 16
[Note to readers: this is from the book Visual Practices Across the University, edited by James Elkins (Munich: Wilhelm Fink Verlag, 2007). This book Visual practices across the university, part two, chapters 1 16 is available on Amazon.$These pdfs were originally posted on the author's website, www.jameselkins.com, and other sites. Please send all comments to the author at iameselkins<@fastmail.fm or via the website.SpectroscopyPierre Laszlo and James ElkinsThe large photograph is a spectrum. It records the Visual practices across the university, part two, chapters 1 16 wavelengths of light, from the deep violet near the end of the visible spectrum (at the top) down into the warmer colors that lead to infrared (at thVisual practices across the university, part two, chapters 1 16
e bottom). Tile machine that made this is a Vrecland spectroscope, a fairly ugly machine that works very simply.F4373E»:nsS_O«.179>nad M1903 2CO7 1122[Note to readers: this is from the book Visual Practices Across the University, edited by James Elkins (Munich: Wilhelm Fink Verlag, 2007). This book Visual practices across the university, part two, chapters 1 16 arbon rods, which arc arranged so they do not quite touch. When the machine is turned on, an arc of electricity sparks between the plates, creating a rasping noise and a blinding light (far brighter than sunlight). The arc melts and vaporizes the rock sample; the spectrum is produced when light is r Visual practices across the university, part two, chapters 1 16 eflected off a diffraction grating inside the machine.When a viewer looks in the eyepiece, he or she secs the spectrum in brilliant colors. The VreelaVisual practices across the university, part two, chapters 1 16
nd spectroscope has two film reels that can run on either side of the spectrum. They can be rolled along using the two black handles visible in the fi[Note to readers: this is from the book Visual Practices Across the University, edited by James Elkins (Munich: Wilhelm Fink Verlag, 2007). This book Visual practices across the university, part two, chapters 1 16 hat are characteristic of different elements. Here the large “Sn" indicates this is the portion of the film that shows the typical lines of Tin. The green line and the blue line match lines in the spectrum, and in fact the powder on the crucible in this case was pure tin from a chemical supplier.F43 Visual practices across the university, part two, chapters 1 16 73EHnsS_O«-179>nda €01903 2007 11 2231 UhtREADING SPECTROGRAMS61From color to monochromeThis is simple spectroscopy, done with a rough-and-ready machiVisual practices across the university, part two, chapters 1 16
ne that was designed to be used in the field, by geologists. Machines like this arc rare inlaboratories now; they have been replaced by massive, and m[Note to readers: this is from the book Visual Practices Across the University, edited by James Elkins (Munich: Wilhelm Fink Verlag, 2007). This book Visual practices across the university, part two, chapters 1 16 involve any color reproductions. This is a series of ultraviolet spectra, numbered 1-8, illustrating an experiment carried out in an organic chemistry laboratory during the 195Cs — that is, almost exactly a full century after Bunsen and Kirchhoff invented spectroscopy.The curves show how the molecul Visual practices across the university, part two, chapters 1 16 es absorb light of different wavelengths. Note that there arc no spectral colors here, and really no colors at all; the original publication had no reVisual practices across the university, part two, chapters 1 16
d dots, and this was its only illustration.U7j.it arc we looking atiThe picture on the top depicts a stack of spectrograms — spectra for short. Each s[Note to readers: this is from the book Visual Practices Across the University, edited by James Elkins (Munich: Wilhelm Fink Verlag, 2007). This book Visual practices across the university, part two, chapters 1 16 of electromagnetic radiation, that is, from 4C0 to 800 mu (a micron, u, is a millionth of a meter, or a thousandth of a millimeter).Why do certain molecules absorb light in that range? Because absorption of a photon lifts an electron — hence the name “electronic spectroscopy” — from a doubly occupie Visual practices across the university, part two, chapters 1 16 d energy level (blue) into a vacant energy level (red).As the light excitation sweeps through the accessible range, in this case from 400 to 650 mp, iVisual practices across the university, part two, chapters 1 16
t probes various electronic energy levels within a given molecule. The manifold of such light absorptions, across the whole range monitored, is termed[Note to readers: this is from the book Visual Practices Across the University, edited by James Elkins (Munich: Wilhelm Fink Verlag, 2007). This book Visual practices across the university, part two, chapters 1 16 an formulas, provide information only about composition. For instance, the methane molecule (natural gas) is CHp benzene C6H( and hydrocyanic acid is HCN. In these, c stands for carbon, H for hydrogen and N for nitrogen.A major step forward was taken during the 1860s when structural formulas were de Visual practices across the university, part two, chapters 1 16 vised. They show in how atoms arc connected. The very same examples arc shown below:Chemists soon realized that rather strict rules governed these forVisual practices across the university, part two, chapters 1 16
mulas. Notice, for instance, how each carbon atom bears a total of four lines (bonds) to neigh-boring atoms? Accordingly, a shorthand was quickly esta[Note to readers: this is from the book Visual Practices Across the University, edited by James Elkins (Munich: Wilhelm Fink Verlag, 2007). This book Visual practices across the university, part two, chapters 1 16 molecule is written.[Note to readers: this is from the book Visual Practices Across the University, edited by James Elkins (Munich: Wilhelm Fink Verlag, 2007). This bookGọi ngay
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