We have next to say something about the principles on which the use of the spectroscope depends; if we look through one we can readily observe how each particular ray of light paints an image of the slit. Thus, if we are dealing with a red ray of light, that ray, after passing through the prisms, will paint a red image of the slit; if the light be violet, the ray will paint a violet image of the slit, and these images will be separated, because one colour is refracted more than the other. Now it follows from this that when the slit is illuminated by white light, white light being white because it contains all colours, we get an infinite number of images of slits touching or overlapping each other, and forming what is called a _continuous spectrum_. Hence it is that if we examine the light of a match or candle, or even the electric light, we get such a continuous spectrum, because these light sources emit rays of every refrangibility. Modern science teaches us that they do so because the molecules—the vibrations of which produce, through the intermediary of the ether, the sensation of light on our optic nerve—are of a certain complexity. In the preceding list of light sources the sun was not mentioned, because its light when examined by Wollaston and Fraunhofer, was found to be discontinuous. Now it is clear that if in a beam of light there be no light of certain particular colours, of course we shall not find the image of the slit painted at all in the corresponding regions of the spectrum. This is the whole story of the black lines in the spectrum of the sun and in the spectra of the stars. Here and there in the spectrum of these there are colours, or refrangibilities, of light which are not represented in light which comes from those bodies, and therefore there is nothing to paint the image of the slit in that particular part of the spectrum; we get what we call a dark line, which is the absence of the power of painting an image. But then it may be asked, How comes it that the prism and the spectroscope are so useful to astronomers? In answer we may say, that if we knew no more about the black lines in the spectra of the sun and stars than we knew forty years ago, the spectroscope ought still to be an astronomical instrument, because it is our duty to observe every fact in nature, even if we cannot explain it. But these dark lines have been explained, and it is the very explanation of them, and the flood of knowledge which has been acquired in the search after the explanation, which makes the spectroscope one of the most valuable of astronomical instruments. Many of us are aware of the magnificent generalizations by which our countrymen, Professors Stokes and Balfour Stewart, and Ångström, Kirchhoff and Bunsen, were enabled to explain those wonderful lines in the solar spectrum.