3. The present report deals with the spectra of the eleven stars enumerated in Table I. Successive columns contain the plate number, the name of the star, its spectral class, the apertures employed, and remarks. In addition to the plates enumerated in Table I, the following focus plates were obtained. ┌─────┬───────────────┬────────────────────────┬──────────────────────┐ │Plate│ Star │ Apertures │ Remarks │ ├─────┼───────────────┼────────────────────────┼──────────────────────┤ │21648│α Canis Majoris│16, 4, 4, 4, 4, 4, 4, 4,│Various focus settings│ │ │ │ 4, 4, 16 │ │ │21803│α Canis Majoris│16, 16, 4, 2, 8, 8, 8, │ „ │ │ │ │ 8, 8, 16 │ │ └─────┴───────────────┴────────────────────────┴──────────────────────┘ 4. All the plates have been analyzed by means of the Moll thermoelectric microphotometer of Harvard Observatory,[3] which furnishes a photographic record of the plate density. The adjustments of this instrument were made with several ends in view. The analyzing beam of light was kept as narrow as possible, so that no integrating effect should enter into the final result. At the same time it was desired that the total galvanometer deflection—the quantity on which the measures depend—should be of reasonable size; otherwise the errors of measurement would become proportionately too great. Some of the analyzed spectra, especially those of fainter stars, were so narrow that the slit admitting the analyzing beam had to be considerably shortened. This cut down the total light transmitted in the same proportion, and to keep the deflections of the galvanometer of reasonable size, a wider slit, and therefore a wider analyzing beam, had to be used. A compromise was worked out, for each plate, between a narrow analyzing beam and a reasonable galvanometer deflection. Footnote 3: This instrument was purchased with the aid of the Rumford Fund of the American Academy of Arts and Sciences and the Bache Fund of the National Academy of Sciences. Special precautions were taken to secure the greatest uniformity of conditions possible throughout the analysis of each series of spectra of any one star. At first it was hoped that the whole series of plates could be analyzed under exactly uniform conditions. Owing to the narrowness of some of the spectra, however, it was necessary to introduce the modifications indicated in the preceding paragraph. Even if all the spectra had been analyzed under precisely the same conditions, experience showed that direct intercomparison between different stars would have been impossible, owing to varying amounts of fog on different plates.