TABLE VI Spectrum 2c 2d 2e 2b 2a Setting 16.2 16.4 16.6 16.8 17.0 _l_ at Hβ 51 52 51 54 53 4481 77 78 79 79 81 Hγ 54 55 56 55 56 Hδ 47 48 51 50 50 Hε 39 39 42 43 41 K 62 64 65 65 64 Hζ 22 26 28 28 26 Hη 8 10 12 12 8 Hθ 4 5 4 6 2 Table VII is in the same form as Table V, and represents the change of _m_ with changing focus. Evidently Spectrum 2c is at best focus. TABLE VII Spectrum 2c 2d 2e 2b 2a Setting 16.2 16.4 16.6 16.8 17.0 _m_ at Hβ 22 22 21 20 19 4481 3 3 2 2 2 Hγ 24 24 23 23 25 Hδ 28 27 26 27 26 Hε 31 31 30 28 30 K 6 5 5 4 4 Hζ 36 33 34 33 34 Hη 32 31 30 30 31 Hθ 20 19 20 19 19 By the use of the four apertured spectra that occur on MC 21803 it is possible to evaluate the differences of intensity, produced by the change of focus, directly in stellar magnitudes. The method used in deriving the intensities is the one employed in compiling Table IX. The intensities at the centers of the lines of the various spectra are summarized in Table VIII. It appears that Spectrum 2c is at best focus for lines at either end of the spectrum, and that the curve of best focus moves towards 2b for intermediate lines. The effect is what would have been anticipated on general grounds. The magnitude of the effect is satisfactorily small, as may be seen by comparing the differences in Table VIII with the residuals in Table IX. Errors arising from bad focus, while they are of appreciable size, do not exceed the errors due to other causes. If the spectra to be analyzed appear upon visual examination to be in good focus, they will probably not give results impaired by serious focus error. [Illustration: Figure 8.—Microphotometer tracings made from Harvard objective prism spectra of Sirius, MC 21648, to illustrate the effects of focus. Analyses are shown of the five lines indicated on the left margin, for the focus settings given above. The best focus is at 16.8; the short lines below the absorption minima indicate the change in line depth with changing focus. The doubling of the K line, and the increasing distance between the components, is a noticeable effect of focus.]