1. In a recent paper on the structure of certain silicates, Prof. W. L. Bragg and one of the authors discussed in considerable detail the role which the oxygen atoms—due to their relatively large size—played in these compounds. The structures examined fell roughly into two classes: one in which the O atoms were throughout packed as closely together as possible, the other in which close packing was local only. Many examples of structures which had actually been worked out were given, and it was suggested that topaz might possibly prove a further example of a close-packed arrangement of large ions. An attempt to solve the structure of topaz on these lines proved unexpectedly difficult, and a detailed general quantitative examination was therefore carried out. A full account of this examination, which has now been completed, will be published later in the 'Zeitschrift für Kristallographie.’ We propose here to give but a general outline of the structure, desiring rather to draw attention to an interesting new type or variant of close packing which the study has revealed, and which suggests important considerations in the elucidation of complex structures still awaiting analysis.

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A primary-standard flowing-water optical power meter for measuring multi-kilowatt laser emission has been built and operated. The design and operational details of this primary standard are described, and a full uncertainty analysis is provided covering the measurement range from 1-10 kW with an expanded uncertainty of 1.2%. Validating measurements at 5 kW and 10 kW show agreement with other measurement techniques to within the measurement uncertainty.

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The C 1s, O 1s, and N 1s X-ray absorption spectra of three lipid species, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), and 1,2-dioleoyl-3-trimethylammonium-propane (chloride salt) (DOTAP), have been recorded using transmission detection in a scanning transmission X-ray microscope. The spectra are presented on an absolute intensity scale (i.e., optical density per nm) to allow their use as reference standards for spectromicroscopic analysis of supported lipid bilayers. Examples of C 1s based spectromicroscopic mapping of saturated and unsaturated domains in dry lipid bilayers of DOPC and DSPC at several compositions are presented. The results are compared with fluorescence microscopy of the same area. Challenges for extending this work to studies of wet lipid bilayers interacting with antimicrobial peptides are discussed.