Note about angle measurement: The angle is computed by dividing the actual angle (recorded by the computer) by two. The best procedure is to set the plastic Brewster's disk to a particular angle and then move the spectrophotometer arm to about the same angle read on the digits display. But, to get the laser beam exactly on to the slit, you must make fine adjustments while watching the digits display for the maximum light intensity. You can adjust either the disk or the spectrophotometer arm until the intensity is maximized.