A gaseous mass, so large as to be opaque, would, if it were of the same temperature inside and out, give a continuous spectrum, without any dark lines. But the laws of temperature in such a mass show that it will be cooler at the surface than in the interior. This cooler envelope will absorb the rays emanating from the interior as in the case when the latter is solid. We conclude, therefore, that the fact that the great majority of stars show a spectrum like that of the sun, namely, a continuous one crossed by dark lines, does not throw any light on the question whether the matter composing the body of the star is in a solid, liquid or gaseous state. The fact is that the most plausible theories of the constitution of the sun lead to the conclusion that its interior mass is really gaseous. Only the photosphere may be to a greater or less extent solid or liquid. The dark lines that we see in the solar spectrum are produced by the absorption of a comparatively thin and cool layer of gas resting upon the photosphere. Analogy as well as the general similarity of the spectra lead us to believe that the constitution of most of the stars is similar to that of the sun. CLASSIFICATION OF STELLAR SPECTRA. When the spectra of thousands of stars were recorded for study, such a variety was found that some system of classification was necessary. The commencement of such a system was made by Secchi in 1863. It was based on the observed relation between the color of a star and the general character of its spectrum. Arranging the stars in a regular series, from blue in tint through white to red, it was found that the number and character of the spectral lines varied in a corresponding way. The blue stars, like Sirius, Vega and α Aquilæ, though they had the F lines strong, as well as the two violet lines H, had otherwise only extremely fine lines. On the other hand, the red stars, like α Orionis and α Scorpii, show spectra with several broad bands. Secchi was thus led to recognize three types of spectra, as follows: The first type is that of the white or slightly blue stars, like Sirius, Vega, Altair, Rigel, etc. The typical spectrum of these stars shows all seven spectral colors, interrupted by four strong, dark lines, one in the red, one in the bluish green, and the two others in the violet. All four of these lines belong to hydrogen. Their marked peculiarity is their breadth, which tends to show that the absorbing layer is of considerable thickness or is subjected to a great pressure. Besides these broad rays, fine metallic rays are found in the brighter stars of this type. Secchi considers that this is the most numerous type of all, half the stars which he studied belonging to it. [Illustration: FIG. 1. SPECTRUM OF SIRIUS.] [Illustration: FIG. 2. SPECTRA OF α AURIGÆ AND SUN.] [Illustration: FIG. 3. SPECTRA OF α BOOTIS AND β GEMINORUM.]