“Spectrum Analysis, then, teaches us the great fact that solids and liquids give out continuous spectra, and vapours and gases give out discontinuous spectra instead of an unbroken light” (Lockyer). We have found out that the sunlight and moonlight are identical, that the moon gives us spectrum like a reflection of the former, but has no atmosphere, and that comets are but gases or vapours. The most minute particles of a grain of any substance can be detected to the millionth fraction. The 1/1000 of a grain of blood can be very readily distinguished in a stain after years have passed. The very year of a certain vintage of wine has been told by means of “absorption,” or the action of different bodies on light in the spectrum. It is now easy, “by means of the absorption of different vapours and different substances held in solution, to determine not only what the absorbers really are, but also to detect a minute quantity.” The application of this theory is due to Dr. Gladstone, who used hollow prisms filled with certain substances, and so thickened the “absorption lines.” By these lines, or bands, with the aid of the Spectrum Microscope, most wonderful discoveries have been made, and will continue to be made. We will close this portion of the subject with a brief description of the Spectroscope in principle. The instrument consists of two telescopes arranged with two object-glasses on a stand (fig. 150). A narrow slit is put in place of the eye-piece of one, the arrangement admitting of the slit being made smaller or larger by means of screws. The glass to which the slit is attached is called the collimating lens. The light, at the end of the slit seen from the other telescope, being separated by the prisms between the two telescopes, will produce the spectrum. The Spectroscope is enclosed, so that no exterior light shall interfere with the spectra the student wishes to observe. This merely indicates the principle, not the details, of the Spectroscope, which vary in different instruments. We may now pass from the Spectroscope to the Telescope and the Microscope, instruments to which we are most largely indebted for our knowledge of our surroundings in earth, air, and water. The word Telescope is derived from the Greek _tele_, far, and _skopein_, to see; and the instrument is based upon the property possessed by a convex lens or concave mirror, of converging to a focus the rays of light falling on it from any object, and at that point or focus forming an image of the object. The following diagram will illustrate this. Let VW be a lens, and AB an object between the glass, and F the focus. The ray, A_c_, is so refracted as to appear to come from _a_. The ray from _b_ likewise appears in a similar way, and a magnified image, _ab_, is the result (fig. 152).