250 LABORATORY NOTES leg of the diffractometer slides in a guide rail mounted on the tail piece and stops are placed on the guide rails to limit the forward and backward movements of the diffractometer. In aligning the diffractometer to the monochromator, the crystal is set to the correct angle for the X-ray wavelength being used, and the exit port set to twice the angle. The diffractometer base is also set to twice the angle of diffract- ion and the diffractometer adjusted by means ot its legs to have the correct height and angle to the horizontal cor- responding to the take-off angle of the monochromator. In such a position the diffractometer collimator should line up with and enter symmetrically the exit port of the monochromator when the diffractometer is slid forward on the guide rails.Thefinal adjustments of mon- chromator angle, diffractometer angle, and diffractometer height can be made very quickly, usirg the detector and electronic counting equipment to max-
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