With the Gian spectrophotometer a quantitative comparison of the absorption spectra of one vibration direction of a crystal plate may be made with an unabsorbed spectra from the same source of light. By repeating the observations, after rotating the crystal to the other extinction position, the other vibration direction may be compared with the same source. The ratio of the two intensities for any given wavelength may then be calculated. Two separate slits are used and the intensity of the light through these two slits is equalized before inserting the crystal plate in front of one of the slits. If the light source fluctuates during the course of the observations so as to change the relative illumination of the two slits, the results are in error. Because minerals are apt to show variation in color distribution or because they may contain irregularly distributed inclusions, another source of error is introduced since the slit necessarily covers two different portions of the crystal plate during the course of the experiment.