5. _Measurement of microphotometer tracings._—In addition to the line representing the density of the image at different points along the spectrum, reference marks were inserted by registering a line for “darkness,” by interposing an opaque screen in the path of the analyzing beam, and a line for “clear film,” by passing the beam through the plate background close to the spectrum, though not close enough to bring it within range of disturbing photographic effects due to the image. The microphotometer tracings on paper prints were measured with respect to the reference marks. Lines, representing “darkness” and “clear film,” were ruled from end to end of the tracing, and across the absorption lines a curve was drawn, completing the curve of the neighboring continuous background. For early type stars this background curve can be drawn without ambiguity; but when the spectrum is rich in lines, the course of the unlined continuous background is largely a matter of judgment. [Illustration: Figure 2.—Diagram of an absorption line, as registered by the microphotometer, showing the method of measuring the tracings. The quantities n (“darkness” to “background curve”), _m_ (“background curve” to “line”), and _l_ (“line” to “clear film”) were measured at intervals of five scale divisions, indicated by the vertical lines.] The quantities measured on the microphotometer tracings are best described by a diagram. Figure 2 represents a wide absorption line, and the various distances that were measured in analyzing such a line. The measures were all made with a half-millimeter réseau scale photographed upon glass, which was laid directly upon the tracing. 6. _Method of reduction._—The spectra obtained with various apertures provide, as was pointed out in Harvard Bulletin 805, several measures of the intensity at any point of an absorption line. The intensity is compared, in the present paper, with the intensity that the continuous background would have at the same point if the line were not present, which is assumed to be represented by the “background curve” drawn across the absorption line. The method has the advantage of making a determination separately for each wave length. The difficulties introduced by the varying color sensitivity of the photographic plate are thus avoided. It has, however, the disadvantage that the measured quantity depends to some extent upon the individual judgment of the investigator in drawing the “background curve”—a matter that is simple for Classes B and A, but may prove serious for second-type stars.