This result is due to the fact that the light of the sun is composed of rays of an infinite number of wave-lengths, or, as we might express it, of an infinite number of shades of color, since to every wave-length corresponds a definite shade. Such a spreading out of elementary colors in the form of a visible sheet is called a spectrum. By the spectrum of an incandescent object is meant the spectrum formed by the light emitted by the object when passed through a refracting prism, or otherwise separated into its elementary colors. The interest and value which attach to the study of spectra arise from the fact that different bodies give different kinds of spectra, according to their constitution, their temperature and the substances of which they are composed. In this manner it is possible, by a study of the spectrum of a body, to reach certain inferences respecting its constitution. In order that such a study should lead to a definite conclusion, we must recall that to each special shade of color corresponds a definite position in the spectrum. That is to say, there is a special kind of light having a certain wave-length and therefore a certain shade which will be refracted through a certain fixed angle, and will therefore fall into a definite position in the spectrum. This position is, for every possible kind of light, expressed by a number indicating its wave-length. If we form a spectrum with the light emitted by an ordinary incandescent body, a gaslight for example, we shall find the series of colors to be unbroken from one end of the spectrum to the other. That is to say, there will be light in every part of the spectrum. Such a spectrum is said to be continuous. But if we form the spectrum by means of sunlight, we shall find the spectrum to be crossed by a great number of more or less dark lines. This shows that in the spectrum of the sun light of certain definite wave-lengths is wholly or partly wanting. This fact has been observed for more than a century, but its true significance was not seen until a comparatively recent time. If, instead of using the light of the sun, we form a spectrum with the light emitted by an incandescent gas, say hydrogen made luminous by the electric spark, we shall find that the spectrum consists only of a limited number of separate bright lines, of various colors. This shows that such a gas, instead of emitting light of all wave-lengths, as an incandescent solid body does, principally emits light of certain definite wave-lengths. It is also found that if we pass the light of a luminous solid through a sufficiently large mass of gas, cooler than the body, the spectrum, instead of being entirely continuous, will be crossed with dark lines like that of the sun. This shows that light of certain wave-lengths is absorbed by the gas. A comparison of these dark lines with the bright lines emitted by an incandescent gas led Kirchhoff to the discovery of the following fundamental principle: