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The use of simple, aperture-collimated cameras in x-ray diffraction studies at small angles is of value in investigating the molecularly large structural features of colloidal systems, particularly when pinhole collimation is required. To make effective use of available x-radiation one must (1) consider the initial limitations imposed on a camera by the purpose for which it is to be employed, and (2) choose remaining camera dimensions so that the registering film may receive as much radiation as possible. The initial conditions are three in number, involving the desired pattern magnification, the necessary angular or large-spacing resolution, and the requisite guarding against beam spill-over, which together essentially limit the specimen-to-film distance, the beam diameter at the film, and the diffraction area permitted to be unguarded but covered by a beam stop. Other camera dimensions are chosen so that the film center is as close to, while viewing as much of, the radiation source as possible. In accomplishing these aims it is useful to align the camera at near grazing incidence to the x-ray tube target, though there are limitations to this procedure. Rules for systematically accomplishing the above purposes are derived herein for the extremes of symmetrical (pinhole) and unsymmetrical (slit) aperture systems, as well as for ones employing a simple class of rectangular (unsymmetrical pinhole) aperture. Criteria are also given for evaluating the relative speeds of cameras which may depart from optimum because of mechanical limitations. Finally is described the construction of a series of cameras, found useful in studying the fibrous proteins, particularly collagen, and typical diffraction patterns obtained therefrom are reproduced.

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The best value (from Levy) for the dissociation constant of the acid R(IH+)NH2FCOO~, is 7'30, and Greenstein gives 9'10 for K>. La turns out to be 4.16 X 104, a figure which is consistent with the characteristics of figure 1. L23 is ¿i3 X 4.52, or 1.45 X 10s.A comparison of the constants calculated by Levy and those given in this paper is given in table 4. In every case in which comparable values are reported, the agreement of the values calculated by the authors with those given previously is quite satisfactory. SUMMARYThe polarimetric method has been applied to the study of the equilibria between histidine and formaldehyde. Contrary to previous reports, evidence is presented to show that each of the three ionic forms of histidine reacts with first one, and then another mole of formaldehyde. The calculation of the values of five equilibrium constants which characterize these reactions has been described. The values of these constants are in good agreement with the comparable values which have been reported previously.REFERENCES (1)

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