b. _Effect of focus._—The effect of poor focus in blurring absorption lines suggests that this factor may enter into the accuracy of the results. It is not possible, in a stellar spectrograph, when working with flat plates, to keep all parts of the spectrum in focus at the same time. Two plates of Sirius were taken for the purpose of examining the magnitude of the effect. The apertures used, and the focus settings, were as follows. PLATE MC 21648 Spectrum 1a 3a 3b 3c 3d 3e 3f 3g 3h 3i 1b Aperture 16 8 8 8 8 8 8 8 8 8 16 Setting 17.2 17.2 17.4 17.6 17.8 18.0 17.0 16.8 16.6 17.2 17.2 PLATE MC 21803 Spectrum 1a 1b 2e 3 4 2c 2d 2b 2a Aperture 16 16 8 4 2 8 8 8 8 Setting 16.6 16.6 16.6 16.6 16.6 16.2 16.4 16.8 17.0 From MC 21648 it is possible to obtain a qualitative estimate of focus effects; MC 21803, including spectra taken with all four apertures, furnishes a quantitative estimate of the magnitude of the focus errors. Microphotometer tracings were made, under uniform conditions, of the spectra of each of the focus plates, and measures were made at the centers of the lines only. For the plate MC 21648, the observing record book contains the entry: “Frost in center of prism at close.” Apparently the frosting resulted in a gradual decrease in the intensity of successive spectra, which is shown, when the spectra are arranged in the order in which they were photographed, by a gradual decrease in _n_, a quantity that should remain constant for the same aperture, since the edges of the spectrum, the portion where focus would affect the intensity, are not crossed by the analyzing beam of the microphotometer. The progressive change in _n_ is shown in Table III. TABLE III Spectrum 3a 3b 3c 3d 3e 3f 3g 3h 3i _n_ at Hβ 13 (12) 13 15 16 16 17 18 17 Hγ 8 8 8 11 9 10 11 10 11 Hδ 11 9 10 12 12 14 12 13 15 Hε 16 14 17 15 18 19 19 19 20 K 19 17 19 18 20 22 21 22 23 Hζ 27 25 28 29 29 31 31 31 33 That the change in _n_ is progressive and not due to change of focus is shown by arranging the columns in the order of focus setting, 3h, 3g, 3f, 3i, 3a, 3b, 3c, 3d, 3e. No regular change in _n_ is then evident.