The Porter experiments

The process of removing specific spatial frequencies by means of an aperture in the y-plane is called spatial filtering. The effects of spatial filtering on the nature of the image of a simple two-dimensional periodic object will now be illustrated by describing a series of experiments (known as the Porter experiments) and by discussing the observations in terms of the concepts already developed. 

By using apertures of various shapes and sizes placed in the back focal plane of LI, we can exclude specific beams and observe the effect on the nature of the image of the grid. 

If N is very large, then only those secondary maxima that lie in a small range of n on each side of each principal maximum of a spectrum have heights which are comparable to that of the principal maximum itself. In 

Finally, we consider the nature of the image formed with a single diffracted beam, say the first-order beam at ii = +no- Under these conditions, the first two terms of Eq. (1.31a) are eliminated and 

To obtain the intensity distribution in the image plane, we must multiply g(,x ) by its complex conjugate 

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The experimental arrangement for the Porter experiments is shown in Figure 1.10. A small helium-neon laser illuminates an object, which is a 3-mm diameter copper grid of the type commonly used for supporting specimens for transmission electron microscopy. The spacing d between grid bars is 125 /tm. 

So far in this discussion of diffraction, we have assumed that the periodic object is illuminated by coherent light, such as that produced by a small laser of the type used in the Porter experiments. However, the light produced by a thermal source (e.g., a sodium vapor lamp or a heated filament coupled with a narrow bandpass filter) is never strictly monochromatic even the sharpest spectral line has a finite width. Moreover, such a source has finite extent, and the light is emitted by many independent radiators (atoms). These two characteristics of thermal sources are directly related to what are usually referred to as temporal and spatial coherence, respectively. 

Porter, Dorothy, and Roy Porter, In Sickness and in Health The British Experience, 16 0-18 0 (1988). 

Rules of Thumb Since in most circumstances packing HETP is sensitive to only few variables, and due to the unreliability of even the best mass-transfer model, it has been the authors experience that rules of thumb for HETP are more accurate and more reliable than mass-transfer models. A similar conclusion was reached by Porter and Jenkins (IChemE Symp. Ser. 56, Summary paper, London, 1979). 

Zewail acknowledged early on that he was inspired to work in the dynamics area by amongst others, George Porter s development of fast reaction techniques, viz. Flash Photolysis which is reported elsewhere in this volume. In the early experiments outlined in the present paper, three detection techniques were employed time-correlated single photon counting, with 30-50 ps time resolution streak camera detection of fluorescence, with 10 ps resolution, and multiphoton ionisation with resolution determined by the pulse width of the laser, 1 or 15 ps. 

We will now consider how to simulate this method of image formation in the X-ray diffraction experiment where we have to use a mathematical replacement for the objective lens. The studies by Porter are of great importance because they show how the Bragg reflections give the amplitude components of a Fourier series representing the electron density in the crystal (the electron-density map). In effect, Fourier analysis takes place in the diffraction experiment, so that the scattering of X rays by the electron density in the crystal produces Bragg reflections, each with a different amplitude F hkl) and relative phase Qhkl-... 

For some mixtures, values of can be extracted from experiment and in those cases we have a convenient means for determining a value for the parameter A. Because the Porter equation (5.6.1) contains only one parameter, a high degree of symmetry exists among the values of g, In and In y2- For binary mixtures, the symmetry appears as in Figure 5.8. 

If the temperature and pressure dependence of the Porter parameter A is known (from experiment), then we can obtain values for and from (5.2.11) and (5.2.12). The results are... 

The next experiment in this series investigated the " bustness of maternal labels as recognition cues for spiny mouse siblings (Porter et al., 1984). That is, will pups anointed with the same maternal label still recognize one another if they are also labeled by two different females, or will the addition of dissimilar maternal labels reduce the effectiveness of the shared chemical signature ... 

Porter RW, Detwiler PW, Spetzler RF, Lawton MT, Baskin JJ, Derksen PT et al. (1999) Cavernous malformations of-the brainstem experience with 100 patients. J Neurosurg 90 50-58... 

Partition chromatography originated with the work of Archer John Porter Martin (6. 1910) in the 1930s. He devised a technique known as countercurrent distribution. The early experiments involved a line of separating funnels containing two immiscible solvents. After shaking the funnels, one of the liquid layers fi-om each funnel was transferred to the next in the line, and the process was repeated. Two solutes travelled at different rates if their partition coefficients for the two phases were different. Subsequently the separating funnels were replaced by special tubes, which meant that the process could be automated. 

Fig. 6.3 Photographic plates of the original experiment by Porter for the identification of benzyl radical. Spectra detected before upper trace) and during the photolysis of (a) benzyl chloride and (b) toluene, showing the appearance of the benzyl radical signals. Reprinted by permission from [19]...

J. Rotter, P. Jumikis, S. Flemming and S. Porter. Experiments on the buckling of thin walled model silo structures. J. of Constructional Steel Research, 13 (1989) 271-299. 

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

Lucas and Porter (U.S. Patent 3,370,401, 1967) developed a fiber-bed scrubber in which the gas and scrubbing liquid flow vertically upward through a fiber bed (Fig. 17-55). The beds tested were composed of knitted structures made from fibers with diameters ranging From 89 to 406 [Lm. Lucas and Porter reported that the fiber-bed scrubber gave substantially higher efficiencies than did venturi-type scrubbers tested with the same dust at the same gas pressure drop. In similar experiments, Semrau (Semrau and Lunn, op. cit.) also found that a fiber-bed contactor made with random-packed steel-wool fibers gave higher efficiencies than an orifice contactor. However, there... 

Irradiation Procedure. Reaction mixtures were prepared at room temperature by transferring desired quantities of reactants from their storage bulbs to the reaction vessel, a 500-cc. spherical borosilicate glass flask attached to the vacuum line by a section of glass capillary tubing and a 4-mm. bore threaded glass valve with a Teflon plug (Fischer and Porter 795-609). Prior to each experiment this vessel was baked under vacuum at 500°C. for 12 or more hours. 

Tanne D, Bates VE, Verro P, Kasner SE, Binder JR, Patel SC, Mansbach HH, Daley S, Schultz LR, Karanjia PN, Scott P, Dayno JM, Vereczkey-Porter K, Benesch C, Book D, Coplin WM, Dulli D, Levine SR. Initial clinical experience with IV tissue plasminogen activator for acute ischemic stroke A multicenter survey. The t-PA Stroke Survey Group. Neurology. 1999 53 424 27. 

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