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309
Laboratory Notes J. Appl. Cryst. (1972). 5, 309
Alignment fixtures for precession cameras Precession camera alignment can be quite critical, especially when one is working with crystals containing large cell spacings. Although alignment errors can normally be detected in the reciprocal lattice photographs, it is usually difficult to determine which part of the instrument is in error, and a new photograph is required after each adjustment. A simple set of fixtures, shown in Fig. 1, has been found useful in checking and correcting the mechanical alignment of cameras. Fixture I consists of a circular plate on a shaft A, which slips into the hole normally occupied by the film holder. Three pointed cones, B, are attached to this plate on a 30 mm radius and located 120 ° apart. By inserting a layer line screen of radius Rs=30 mm and positioning it against the fixture, the centering and parallelism of the layer-line screen holder can be confirmed. Fixture III is a pointed shaft, identical in diameter to the collimator, which is inserted in the collimator bracket. When positioned properly the point should touch C, the center indicator of I. Fixture II is attached to the spindle of the camera via the standard goniometer head base D, and translated until the
J. AppL Cryst. (1972). 5, 309
A simple device for pulling
glass mounting f i b e r s The pulling of mounting pins from lengths of 2 mm glass rod invariably leads to many off-center and bent fibers. A simple device, shown in Fig. 1, can be employed to save many frustrating hours by consistently allowing construction of accurate pins of virtually any desired fiber diameter. The device consists of two parallel ¼ inch ground rods A, with a pin vice B held in a movable mount C. One end plate has an 8 mm glass rod D, which is coaxial with the pin vice and held by the nylon screw E. In our version, roller bushings F were used, but bronze bushings would probably suffice. The 2 mm rod is precut into 7 mm lengths and inserted into the pin vice. The exposed end is then heated by a torch on low flame until melted, is touched to the
two cones E touch the shaft of the collimator fixture II1. Rotation by 180 ° will indicate whether or not the spindle axis is perpendicular to the collimator axis. Fixture IV is simply an adaptor to allow insertion of III into the film-holder hole, as a second check on the centering of the collimator. Two other fixtures have been constructed similar to II. One consists of a movable crosshair on a goniometer base which is used to insure that the collimator and spindle axis intersect, and the other, which is mounted on the same base, is a fluorescent screen so that both hands can remain free while aligning the camera at the X-ray port. Details of the fixtures for Charles Supper and Jarrell Ash precession cameras are available from the author. JOHN C. HUFFMAN
Department of Chemistry Indiana University Bloomington Indiana 47401 U.S.A. (Received 30 March 1972; accepted 2 April 1972)
Fig. 1. Alignment cameras, fixtures for precession
8 mm rod, and translated along the movable track. After several practice attempts, fibers of 0.2 mm and larger diameter can be pulled with ease. For smaller diameters it is necessary to first form a fiber of about 0.5 mm diameter, then to re-heat near the 2 mm rod and
Fig. 1. Device for pulling glass fibers
pull again. In this manner fibers of only a few microns in diameter can be formed. The fiber thus constructed can be inserted in a previously drilled -~ inch brass pin for insertion in the goniometer head, although the latter could be eliminated by using larger diameter glass rod. Experience has shown that at least two feet of travel should be allowed when constructing the device, and it is best not to use glass-blowing glasses in order to best judge the heating of the glass. Construction details are available from the author upon request, JOHN C. HUFFMAN
Department of Chemistry Indiana University Bloomington Indiana 47401 U.S.A. (Received 30 March 1972; accepted 2 April 1972) ....
