The intensity differences, background _minus_ line, were determined for several points by direct measurement. The distances, _n_ and _m_ + _n_ for the same wave length in all the spectra of any one series, were obtained from the microphotometer tracings, and were separately plotted against the logarithms of the corresponding apertures. Smooth curves were drawn, joining the plotted points for any one wave length, as in Figure 3. The drawing of the curves is somewhat simplified by considering together several for the same star, remembering that the sections lying between the same abscissae should be roughly parallel. These various curves represent different sections of the familiar characteristic curve for photographic blackening, the logarithm of the aperture being here substituted for the more usual logarithm of the intensity. Differences of intensity between line and background are then readily obtained by interpolating values of n on the curve connecting _m_ + _n_ and aperture, and similarly by interpolating values of _m_ + _n_ on the curve connecting aperture with measured values of _n_. Each spectrum thus furnished at least one, and sometimes two, values for the intensity difference at any point. It will be seen from Table IX that for several stars two mean values of line intensity are given, one being the mean of all the measures, and the other the “selected mean.” The selected means are obtained by using only points from the more linear portions of the characteristic curve, and by rejecting values derived from microphotometer tracings of exceptional total deflection. [Illustration: Figure 3.—Relation between galvanometer deflection (representing plate density) and aperture (representing light intensity), from measures of the microphotometer tracings made from the apertured spectra of α Lyrae, MC 20790. Ordinates are galvanometer deflections in scale divisions, abscissae are (above) apertures, (below) logarithms of apertures. Smooth curves are drawn joining the points corresponding to the same wave length, for the three apertures represented.] The intensity drop from background to line is thus obtained in the form log. intensity of background _minus_ log. intensity of line. The change in intensity may readily be converted into stellar magnitudes by dividing the difference of the logarithms by 0.4. 7. The results embodied in the present paper differ so materially from those of some previous workers, that it is of especial interest to examine the accuracy that may be claimed for each stage of the work, and the weight that may be assigned to the results, (Cf. Harvard Monograph No. 1, p. 51). Three stages of the investigation should be considered separately; the plates, (a and b), the microphotometer records (c), and the measures (d).