159. _Effect of Increasing the Power of the Spectroscope upon the Brilliancy of a Spectrum._--An increase in the power of a spectroscope diminishes the brilliancy of a _continuous_ spectrum, since it makes the colored band longer, and therefore spreads the light out over a greater extent of surface; but, in the case of a _bright-lined_ spectrum, an increase of power in the spectroscope produces scarcely any alteration in the brilliancy of the lines, since it merely separates the lines farther without making the lines themselves any wider. In the case of a _reversed_ spectrum, an increase of power in the spectroscope dilutes the light in the spaces between the lines without diluting that of the lines: hence lines which appear dark in a spectroscope of slight dispersive power may appear bright in an instrument of great dispersive power. 160. _Change of the Spectrum with the Density of the Luminous Vapor._--It has been found, that, as the density of a luminous vapor is diminished, the lines in its spectrum become fewer and fewer, till they are finally reduced to one. On the other hand, an increase of density causes new lines to appear in the spectrum, and the old lines to become thicker. 161. _Change of the Spectrum with the Temperature of the Luminous Vapor._--It has also been found that the appearance of a bright-lined spectrum changes considerably with the temperature of the luminous vapor. In some cases, an increase of temperature changes the relative intensities of the lines; in other cases, it causes new lines to appear, and old lines to disappear. In the case of a compound vapor, an increase of temperature causes the colored bands (which are peculiar to the spectrum of the compound) to disappear, and to be replaced by the spectral lines of the elements of which the compound is made up. The heat appears to _dissociate_ the compound; that is, to resolve it into its constituent elements. In this case, each elementary vapor would give its own spectral lines. As the compound is not completely dissociated at once, it is possible, of course, for one or more of the spectral lines of the elementary vapors to co-exist in the spectrum with the bands of the compound. It has been found, that, in some cases, the spectra of the elementary gases change with the temperature of the gas; and Lockyer thinks he has discovered conclusive evidence, in the spectra of the sun and stars, that many of the substances regarded as elementary are really resolved into simpler substances by the intense heat of the sun; in other words, that our so-called elements are really compounds. Chemical Constitution of the Sun. 162. _The Solar Spectrum._--The solar spectrum is crossed transversely by a great number of fine dark lines, and hence it belongs to the class of _reversed_ spectra. These lines were first studied and mapped by Fraunhofer, and from him they have been called _Fraunhofer's lines_. [Illustration: Fig. 179.]