Previous research indicates that perceived orientation and/or alignment of segments and points can vary as a function of the angular position of the stimulus elements. Several studies show that the variability of the responses is least and accuracy of judgment is greatest where segments and dots are aligned with a cardinal axis. Additionally, some report assimilation of judgments toward the nearest cardinal axis--that is, the segments (or dots) are seen as being closer to the horizontal or vertical than is true. The present research confirms that judgments of collinearity are least variable and most accurate when the segment being judged is aligned with a cardinal axis. However, we do not find any consistent tendency for cardinal axis assimilation. Plotting the collinearity error (delta) as a function of angular position (phi), we find a distinctive profile of oscillation for each subject. Furthermore, subjects who were evaluated in two sessions showed very similar profiles of delta oscillation from Day 1 to Day 2. Harmonic analysis indicated a wide-ranging pattern of significant components. The components at the 4th harmonic and below were more likely to be significant, but each subject showed differential loadings in terms of which of the components were significant, as well as in the sign and amplitude of significant components. These results may reflect idiosyncratic fixation tendencies, or individual differences in the design of neural mechanisms that encode the angular positions of stimuli.

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Optical absorption over a large dynamic range obtained by two different experimental techniques is reported for an undoped a-A1 2 0 3 single crystal. Absorption data are presented in two energy ranges: 0.94-3.5 eV and 4.5-8.6 eV. Vacuum ultraviolet (vuv) absorption measurements were performed on three bouies of Czochralski-grown single-crystal Al 2 0 3 with differing starting material purities. The initial powder purities were 99.99%, 99.999%, and 99.999999%, In addition to the vuv measurements, calorimetric absorption results obtained in the visible and near-infrared (ir) are presented. An empirical formula is obtained that provides an estimate of the absorption coefficient from the near-ir to the vuv for undoped Al2 0 3 • Detailed impurity analyses and sample histories are given for the optical-quality a-A1 2 0 3 utilized herein.