_Type A_ (Sirius type) is characterized by the great intensity of the hydrogen lines (compare plate III). The helium lines have vanished. Other lines visible but faintly. The greater part of the stars visible to the naked eye are found here. There are 1251 stars brighter than the 6th magnitude which belong to this type. Sirius, Vega, Castor, Altair, Deneb and others are all A-stars. _Type F_ (Calcium type). The hydrogen lines still rather prominent but not so broad as in the preceding type. The two calcium lines H and K (396.9, 393.4 μμ) strongly pronounced. Among the stars of this type are found a great many bright stars (compare the third chapter), such as Polaris, Canopus, Procyon. _Type G_ (Sun type). Numerous metallic lines together with relatively faint hydrogen lines. To this class belong the sun, Capella, α Centauri and other bright stars. _Type K._ The hydrogen lines still fainter. The K-line attains its maximum intensity (is not especially pronounced in the figure of plate III). This is, next to the A-type, the most numerous type (1142 stars) among the bright stars. We find here γ Andromedæ, β Aquilæ, Arcturus, α Cassiopeiæ, Pollux and Aldebaran, which last forms a transition to the next type. _Type M._ The spectrum is banded and belongs to SECCHI's third type. The flutings are due to titanium oxide. Only 190 of the stars visible to the naked eye belong to this type. Generally they are rather faint, but we here find Betelgeuze, α Herculis, β Pegasi, α Scorpii (Antares) and most variables of long period, which form a special sub-type _Md_, characterized by bright hydrogen lines together with the flutings. Type M has two other sub-types Ma and Mb. _Type N_ (SECCHI's fourth type). Banded spectra. The flutings are due to compounds of carbon. Here are found only faint stars. The total number is 241. All are red. 27 stars having this spectrum are variables of long period of the same type as Md. The spectral types may be summed up in the following way:-- White stars:--SECCHI's type I:--Harvard B and A, Yellow " :-- " " II:-- " F, G and K, Red " :-- " " III:-- " M, " " :-- " " IV:-- " N. The Harvard astronomers do not confine themselves to the types mentioned above, but fill up the intervals between the types with sub-types which are designated by the name of the type followed by a numeral 0, 1, 2, ..., 9. Thus the sub-types between A and F have the designations A0, A1, A2, ..., A9, F0, &c. Exceptions are made as already indicated, for the extreme types O and M.