On plate III, where the classification of the stellar spectra according to the Harvard system is reproduced, will be found also the wave-lengths of the principal H and He lines. By the visual spectrum is usually understood the part of the radiation between the FRAUENHOFER lines A to H (λ = 760 to 400 μμ), whereas the photographic spectrum generally lies between F and K (λ = 500 to 400 μμ). In the earliest days of spectroscopy the spectra of the stars were classified according to their visual spectra. This classification was introduced by SECCHI and was later more precisely defined by VOGEL. The three classes I, II, III of VOGEL correspond approximately to the colour classification into white, yellow, and red stars. Photography has now almost entirely taken the place of visual observations of spectra, so that SECCHI's and VOGEL's definitions of the stellar spectra are no longer applicable. The terminology now used was introduced by PICKERING and Miss CANNON and embraces a great many types, of which we here describe the principal forms as they are defined in Part. II of Vol. XXVIII of the Annals of the Harvard Observatory. It may be remarked that PICKERING first arranged the types in alphabetical order A, B, C, &c., supposing that order to correspond to the temperature of the stars. Later this was found to be partly wrong, and in particular it was found that the B-stars may be hotter than those of type A. The following is the temperature-order of the spectra according to the opinion of the Harvard astronomers. _Type O_ (WOLF-RAYET stars). The spectra of these stars consist mainly of bright lines. They are characterized by the bright bands at wave-lengths 463 μμ and 469 μμ, and the line at 501 μμ characteristic of gaseous nebulae is sometimes present. This type embraces mainly stars of relatively small apparent brightness. The brightest is γ Velorum with _m_ = 2.22. We shall find that the absolute magnitude of these stars nearly coincides with that of the stars of type B. The type is grouped into five subdivisions represented by the letters Oa, Ob, Oc, Od and Oe. These subdivisions are conditioned by the varying intensities of the bright bands named above. The due sequence of these sub-types is for the present an open question. Among interesting stars of this type is ζ Puppis (Od), in the spectrum of which PICKERING discovered a previously unknown series of helium lines. They were at first attributed (by RYDBERG) to hydrogen and were called “additional lines of hydrogen”. _Type B_ (Orion type, Helium stars). All lines are here dark. Besides the hydrogen series we here find the He-lines (396, 403, 412, 414, 447, 471, 493 μμ). To this type belong all the bright stars (β, γ, δ, ε, ζ, η and others) in Orion with the exception of Betelgeuze. Further, Spica and many other bright stars. On plate III ε Orionis is taken as representative of this type.